Hello everyone,

I'm new to this forum, and need your help on a TH400 concern I have. I've got a 1979 K10 shortbed, completely restored, with a 350/TH350/NP203 setup, that I want to replace with a TH400/NP205 setup.

I found a great condition NP205 for a TH400, and now I found an 87 K20 with a THI 350/TH400/NP208 setup. My point is, this truck has the 85 and newer hump stlye (W shaped) transmission crossmember, instead of the 84 and older flat crossmember. Since the adapter plate is different, shorter with this crossmember, can somebody tell me if GM was stupid enough to change bolt pattern of the housing where the adapter plate mounts to the transmission? I,ve noticed on Ebay TH400 that the seller mentioned it would fit shorter, 85 and up NP208 and NP205 tcase only.
Is the bolt pattern, output shaft of 85 and up TH400 different than older TH400? If I install an 8 figure, old adapter plate, will it bolt right in to the TH400 and NP205? I could get this 87 TH400 real cheap, but I don't want to start to mix and match too much.

Thanks a lot!  

Why do you not take this to "next" level. Forget the THM400. Let me explain. THM400 was a good tranny in its day but that day is long gone. A 400 has a 2.48 first, 1.48 second and a 1.0 drive (with no converter lockup ever) and a tall 2.07 reverse. This means with say 4.10's (which is not really a good gear with big rubber)your maximum crawl ratio is 2.48 x 1,96 x 4.10 or about 20 to 1. (you can throw in 2x more for converter stall but it is not constant or sustainable) Now consider a 700R4 with a NP208 and say 5.13 gears. your max crawl ratio is 3.06 x 2.6 x 5.13 or about 41 to 1 without converter stall. Big difference. Then there is reverse, max crawl in reverse is 4.1 x 1.96 x 2.07 or about 16.7 to one vs 5.13 x 2.6 x 2.29 or about 30.5 to one. Then there is cruise RPM, with THM400 combo and say 35's your cruise RPM will be about 2700 to 2800 (reason for range is that converter has a 5 to 7% slippage). With 700R4 combo at same speed you are turning about 2300 RPM in OD and have a bunch more power in lower gears. Some give 700R4 bad raps but that is because they put big rubber on truck with stock gears and slow back tranny in OD. (if you ran a 700R4 in the "3" position and OD locked out, it would be the same as a THM350 with a deeper first and second gear. THen there is the Tcase. Many swarm to 205 saying they are stronger (and they are) but then same people do not believe in regearing axles properly so they place a lot of extra strain on Tcase. If you gear axles properly with a NP208 you will not have any problems with one as they are pretty reliable when you do this. It is easy to find a 208/700R4 combo and since that combo was used in the truck in mid 80's it is not hard to bolt in either. Lets briefly look at high range 2wd or 4wd. With THM combo you have 2.48 x 4.1 or about 10.2 to 1 plus stall at standstill. With 700R4 combo above you have 5.13 x 3.06 or about 15.7 to 1 before stall (or about 50% more available wheel torque). Sorry if I got off on a tangent here but I come from old school with 4x4's and believe in serious gears in them so they look AND play the part. So many build 4x4's with limited 3 speed auto's and do not consider the possibilities a 4 speed auto can offer when used properly.  

the bolt pattern was never changed.
the difference may be the length ot the output shaft.