You commented on an earlier post about the 66 C6 which we both have. I rebuilt and updated my C6 myself and have a lot of pictures and information on updating the valve body and other parts. I had another C6 I bought which had recently been rebuilt but wasn't able to use because at the time I thought all were the same. I ended up with a 460 bell housing C6(paid $80) and didn't find that out until a year later when I was ready to rebuild the engine. It turns out that trans was a blessing it was a 80s model C6.

I have info that one wants a vb from 1976-up with a cast number starting with D6, mine starts with D7(my 460 C6). To identify the 1976-up vb in addition to casting number D6, the lack of spring or detent groove on the manual valve, and a large tooth on the casting. On my 66 C6 and possible yours if you switch out the vb you will want to take your manual valve which HAS grooves and put in the newer vb if you keep your 66 case because the 66 case DOES NOT have the detent spring and roller in the case like the 1967-up does. Otherwise your gear lever inside the car could easily slip in different gears.

2) If you keep the 66 case DO NOT trade out your inner race or align the oil hole according to Hayes Auto Trans Book along with other books or videos, all are based on 1967-up. The 66 version DOES NOT have the oil hole drilled in the case to align. You can easily drill the hole to update the case. My inner race was fine so I didn't update the case at least not this time.

3) The 1976-up vb has check balls at locations my 66 doesn't(will provide a pic link later). The newer vb I'm using has a low-reverse piston with NO relief ball(unlike the 66 piston) as the added ball in the newer vb does this job. You can use the early piston with relief valve with newer vb but not an older vb(like my 66) with the newer piston. I used the newer piston with the updated vb. If you use the piston with no relief with a one-ball vb(like my 66 vb) the pressure stays on the piston stays applied in all ranges after reverse is selected and the low-reverse clutch pack goes up in smoke.

4) The 1966 C6 has an over sized/heavy duty anchor strut that that goes between the intermediate band and band adjusting screw. You will need to use the 1967-up smaller anchor in place of the 66 one because it is too wide and prevents the extra tooth the newer vb has. After I finished with everything and was ready to install the cleaned newer vb, it would fit because of the original 66 wide anchor. The slimmer fits perfectly the same, it's just slimmer.

5) My 66 C6 was/is heavy duty, maybe because mine have the 428 Q engine. If you do just get a 1967-up case I'd recommend taking parts or checking to see if yours have heavy duty parts installed. Here is what my 66 had:
A) Rare four-pinion rear planetary as opposed to the normally installed three-pinion(66 an upgrade). Although it's a plus it's only needed in the highest horsepower or high-load applications. Failures are almost non-existent with the four-pinion one.
B) High clutch drums(66 an upgrade): I found through my research/build that three-clutch drum is the standard item and is good for most applications up to 400hp. This is the drum in the 460 C6 with no oil holes drilled. However my 66 drum had five with the drilled oil holes. Performance builders add/drill the oil holes and buy after market 4 and 5 drums. It's recommended for over 400 hp a 4 clutch drum be used. Early high clutch drums (my 66 C6) were drilled at four locations to help evacuate oil when the clutch applied and to deliver extra lube to the then cast band used. When the flex bands (currently used) came into use, these holes were deleted and considered by Ford no longer necessary. You want to use the drilled drums with cast bands. Flex bands work with either and why performance builders drill the extra holes like the earlier drums had.
C) Output Hubs(66 an upgrade): came in three flavors. The 1987-up style is used with a rear bearing. The 1969-86 version style uses a three-tang washer (like the 460 C6). The 1966-68 version uses a two-tang washer(my 66). You can use the bearing from the 1987-up unit on any of the hubs to upgrade them, but you must use the short-style race as well. **Hub spines can be a weak spot and the longer spines(my 66) are preferred. The 1966-68 part has the longest followed by the 1987-up and then the 1969-86 three-tang.

Other upgrades/changes I did:
1) The early piston uses a square-cut seal(my 66). I used the later part from the 460 C6 that uses a lip seal so the piston is back-cut to accent it.
2) Replaced my sun assembly with a new low ratio E40D sun gear along with the E40D front planetary and gear as a set for better low-end performance.
3) Front pump to direct drum thrust washers are either steel(my 66) or plastic(460 C6). Both work but the plastic is slicker and does not shed material. I updated to the plastic. They come in different thickness (1 - 5 five thickets) to get the proper end play desired.
4) I used the original servo instead of the 460 C6 because the 1966 C6 servo piston was larger.
5) I went with the high performance Alto Eagle red and steels.
6) I used the 460 C6 forward drum as opposed my 1966 because the 460 C6 had 5 friction disk as opposed the 4 in my 1966 C6. Before buying a five-clutch drum you should know that five-clutches are required for the most ferocious street or RV applications.
**FYI: Because I used the 460 C6 forward drum, I also had to use the shaft from the 460 C6. From 1966-70 these drums used a 31-spline drum and shaft(like my 66) but the 1971-up units had 30 spines(460 C6) so they can't be mixed.

**FYI: Had I known what to look for in the different C6s or if it was a difference in them I wouldn't had bought the 460 C6 thinking it would fit my engine, although it turned out to be a plus for me. So here's some useful info. The FE housing measures 6" apart from the two top bolt holes, the big-block 335(460/429) measures 7 3/4" and the small block housing measures 5 1/8".

I know that everyone who owns one is not going to build one like I did, but knowing about these parts and techniques will make it possible for one to determine how good your builder is and may add to his knowledge and improve what he builds. Hope this is helpful to this group.

UTC428, Thank for the information! My stock C6 is holding up fine, so far, but I will incorporate many of the updates you describe when it's time to rebuild. I have printed out your post and will keep it on file.