Need some help / 400 VS 350 Transmission

[remist17]

Well I bought a 87 stepside a little while ago and will be swaping in a 383. I thought I would get a built replacement trans. I looked at the spec sheet in the glove box and saw that it was a 3 speed. Being stupid and in a rush assumed that it was a T350. I ordered the 350 and it is in. Well I put the truck on the lift last night to take the bed off and noticed a funny looking trans pan.... Well I feel stupid and needless to say the trans is not a 350 but a 400.

The difference between the trans are 2". I believe the driveshaft is 2" shorter than I need. Is there any places that I can get a good driveshaft that is already built? I assume this should not be to hard to find as these trucks had a 350 in them. My searches are not coming up well. Any help is appreciated....

[jrcaprai]

You can take the Driveshaft you have, and get it lengthened at any driveshaft shop, and it is fairly reasonable,

[Desert1957]

I have an 87 Long Bed , it does not have a 350 or a 400 , but its both. Its a Turbo "375". Here is an earlier post about this transmission.

This is my understanding of the transmissions in the 87's and I don't know whatever other years. Many years ago (As told by my transmission Guy) Gm ran short do to a strike , or parts problem with their Longshaft "350 Turbo's" during the middle of the production run. They scrambled for an answer. They retro fitted there "400's" with the longer mainshaft / yoke and tail stock to fit into these trucks.
This solved many problems. First of all the crossmember could remain the same and the front driveshaft / yoke remains the same as a Longshaft "350 Turbo".


Desert

[remist17]

Thats interesting information. I put a tape to the two trans and the 400 is longer by 2.75"
I called around to local shops and only one would make a shaft. My concern is my stock shaft seems small for 400hp.... Looking at after market shafts that are garunteed to handle 500 seem larger.

[benoit454]

might find one local from a long bed or something so it's longer then what you need and have it cut down, it's usually cheaper then having one made. It's gonna cost alot to have one built to handle hp and such.

[OleBlue79]

Take one out of a long wheelbase truck and have it shortened to fit. The long wheel base driveshafts have a larger diameter. You also need special crossover u-joints. They make some that are for a TH-400 on one side and a TH-350 on the other side. You'll need them to connect the driveshaftb to yur new transmission.
And a little free advice if I may. These trucks are old and many of them have swapped hands several times. Everybody tinkers with them and changes stuff. Always verify with your own eyes what you are dealing with.

[remist17]

Well I took a little closer look at the trans last night. I dropped the pan and it looks great. The filter was clean and fluid looked new. Instead of spending the money right now I am going to put a shift kit in the 400 and run it. That way I will save up the money and get the correct one made for the 350 later down the road.
Also just realized the entire truck has 50,140 miles on it. I thought it had 150K. The truck was exempt from miles so the title did not have any. Again I assumed and was wrong. But this is a plesant surprise.

[OleBlue79]

I was fooling with the idea of installing a 700R4 in my 79 C-20. I went to a local driveshaft shop and he said he could shorten and balance a drive shaft for 100 bucks. Not too bad. It would be easier and cheaper to shorten one than to try to lenghten one. You may still have 150K on it since the odometers did not have the 6th digit. When they got 99,999.9 they just rolled over and started at 00,000.0. Mine was showing 5000 miles on it when I got it. God only knows how many times its zeroed itself. Good luck with your project.
A little more advice. I would reuse the same fluid in the trans. Alot of times, if they haven't been serviced regularly and there are a lot of miles on them, they will burn up with new fluid. I do not know why. I just know they will.
If you want to improve your shifts with the TH-400 or a TH-350 without spending any money, you can screw in on the little screw inside the vacum modulator valve. Don't go over 4 rounds. I went in on mine 3 rounds and it shifts a lot better. Takes a little more RPM to get it to shift.

[78 Chevyrado]

If you can find a driveshaft shop, it may even be cheaper then a ready built shaft. I know to shorten my 2 piece on my 3/4 ton, the guy only wanted $40. I'd imagin making it longer would be a bit more but not much.

[OleBlue79]

Hey 78, I got my rims and tires on today. Looks great!