TH400 rebuild advice

[77crewcab]

Well my th350 is finally slipping like crazy and I have decided to be on of the adventurous ones to dive into my first tranny rebuild towards the end of January. I am going to be rebuilding a th400 but need some questions answered.
1. Do I want a short or long tail 400?
2. Should I go ahead and replace the flexplate as well?
3. Other than a rebuild kit (Northern Auto Parts #K3400D-R) and the corresponding shift kit and tech manual, new converter, and speedo gear. What else might I need?
4. Is the length different from the 350 to 400, will I need to have my drive shaft shortened?
5. Will the 350 shift linkage work, or do I need to find a 400 linkage?
6. I understand I will probably have to move the trans crossmember, but the holes should already be drilled right?

I know it is a lot of questions but I want to be prepaired for this little adventure. Thanks!

 

[1979jimmy350]

as to the tail shaft question is this a 2wd or 4x4?
Unless the flex plate is cracked it will be fine
the other parts that your would need will depend on the shape that your core tranny is in. What all comes in the kit? Because some do not have every thing that you would want for the rebuild

 

[77crewcab]

re

2wd crewcab, Rebuild kit description states "Transtech paper and rubber kit with sealing rings, seals, new steels (not tumbled) and Raybestos friction clutches, adjustable modulator, front band, filter and popular bushings and washers." Core would be complete and possibly even driveable but in need of rebuild.

 

[1979jimmy350]

you will need the rear band also. The tail shaft length depedns on what tail shaft is on the 350 there are charts on many sites with the lengths of trannys. Your best bet would to go with the tail shaft that makes the tranny as close to the same length as the 350 that you have.

 

[diesel4me]

More things to know

You'll need a different tranny mount(TH400 differs from the TH350 slightly)--and the TH400 has a different overall length,so it may well be the driveshaft will need to be altered--and the slip yoke on a TH400 has larger splines than the TH350--you could use a conversion U-joint to put a TH 400 yoke on the Th350 style driveshaft...probably have to move the crossmember to the other set of holes in the frame(if there drilled already,some arent)---
Thats all I can think of offhand---sorry,I dont know too much about rebuilding automatics--if I did I'd be building a TH350 out of one of the cores I have sitting in my shed for my truck!...

 

[77crewcab]

re

Well I looked at the rpo sheet and it says the truck came with the M40 which is the 400 right. So the cross member holes sould be there and since I have the two piece drive shaft I would only have to have the smaller piece shortened is that right. That would mean the shift linkage is for the 400 already as well. HMM now to figure out if I should do the job myself or just get one already rebuilt. The price isn't that much different and with a 15mo/15k warranty for it, verses no warranty if I do it myself. All for about $100 difference. funny thing is I don't have the kickdown switch on the gas pedal and if it origionallly had a 400 it should.