I have an 84 K20 that currently has a th350/np205 set up. The trans is bad so I'm looking for a 400. My questions are; What mods will needed to be done? If it bolts right up will I need to get an adapter for the tans case? Would I have to make any changes to the drive shaft or the trans. itself? What about gearing? How do I know what spline my 350 is with out taking the trans off? I have never delt with transmissions before. If anybody can give me some advice it would be great.. Thanks in advance. :confused:

You need a TH400 with the NP 205 compatible output shaft and the NP 205 to TH400 adapter. The driveshafts will need to be shortened in the rear and lengthened in the front. The TH400 is 4 or 6 in longer. You will need to modify the NP205 shifter, it hits the seat in 81 and up cabs with a bench. Sounds like alot but its worth it.

You need a TH400 with the NP 205 compatible output shaft and the NP 205 to TH400 adapter. The driveshafts will need to be shortened in the rear and lengthened in the front.

Is this something that can be done in my garage or would it be best to take to a shop? The 400 is married to a 203 now, would it be better to use that instead of the 205?

No the 205 is better. 203 is chain drive and will require new driveshafts. I have complete 400/205 if you were close it might be worth getting the complete setup.

Edit: I just see you have already inquired about it. You will need a 32spline input as well as the adapter but that should allow you to use the 205 you have.

Can I check the spline with out taking things out? I'm in AZ which isn't exactly close to NE. Know where I can get said adapter for cheap?

Th350/700r4/4l60e all have a 27 spline output
th400 have a 32 spline.
SM465s with the racetrack adapter have the 10 spline
sm465 with a roundfaced adapter have a 32 spline.

fair enough?

Helps me out alot... thanks Old Fart

I just called a local transmission shop and they're telling me i can't put in a 400 cuz of the 205? Does anyone know differantly

Advance Adapters has everything u need but they are proud it. Just FYI as a last resort.

The th400 will change your d-shafts by 3," add to the front, remove from the rear, assuming you keep the same t-case. Just did the swap myself not too long ago.

if you don't mind what did it cost.... pm me if you want

wait!

dont let anyone tell you that you cant put a 205 in behind a th400.

it was a factory part.

you need the adapter.
you need the output shaft, normal short shaft for CAR with tailhousing
you need the input gear.
you need to machine the NP205 case for the larger bearing.

to buy the input gear for the transfer case, possibly the hardest part of the whole swap, you go to http://www.tbtrans.com/transfer_case.htm...scroll down to NP205 for GM and get the 7 inch long input shaft, Part Number 13929 for 97 bucks, and have the transmission shop install it into your GM NP205. Screw Advance Adapters and their buttfuck of a price. Go onto ebay and buy the adapter. theres others out there that are aftermarket too.

dont pay through the nose.

Thanks for all the info... Since I lack the knowledge to do the work it would cost me alot... Think I'll just pick up a rebuilt 350 and bolt it in.