Transmission swap questions

86c10kid · Thursday at 10:50 PM

Hi, so I’ve got a few questions about transmissions and stuff

My 700r4 gave out recently and im trying to decide what to do. My neighbor has a 700r4, th350, and th400 that he would sell to me. I would like to go with the 350 or maybe the 400 unless y’all think I should stay with the 700r4. Let me know your thoughts/opinions. Or send a link to another post if this is already talked about. Thanks in advance

Ricko1966 · Thursday at 11:05 PM

There is more than one version of all of those transmissions. What year is his 700r4? What tailshaft length on the TH350 and TH400? Biggest killer of 700r4s is misadjusted TV cable.Were you messing with the cable or did a carb swap?

xm20k · Thursday at 11:14 PM

Tail shaft lengths require different driveshaft lengths, crossmember may sit in different position, connectors, linkage, lots of things could be an issue. Need more info.

ali_c20 · Friday at 12:41 AM

Crossmember needs to be moved. Driveshaft could fit if the overall length is the same or very close.
Which year is the 700r4 you could buy?

700r4 info:

https://www.gmsquarebody.com/threads/how-to-identify-a-700r4.1865/

Chart with the transmission lenghts:

You must be registered for see images attach

Rickf · Friday at 9:44 AM

Depending on the budget, I would go with a TH400 + a GV overdrive. It's probably the most expensive option, but it would be bullet proof.

86c10kid · Friday at 9:58 AM

Ricko1966 said:
There is more than one version of all of those transmissions. What year is his 700r4? What tailshaft length on the TH350 and TH400? Biggest killer of 700r4s is misadjusted TV cable.Were you messing with the cable or did a carb swap?

Yes I was kinda messing with it and realize I probably burnt up the transmission. It had the stock 1986 700R4. The one my neighbor has is 1984. The th350 is 1970ish, the 400 is 1979

Ricko1966 · Friday at 10:02 AM

86c10kid said:
Yes I was kinda messing with it and realize I probably burnt up the transmission. It had the stock 1986 700R4. The one my neighbor has is 1984. The th350 is 1970ish, the 400 is 1979

Not enough info there. You need to know tail shaft lengths on the th350 and 400. At least find out what the THs were out of. The 1984 700r4 is not a very desirable 700r4 unless it's cheap and guaranteed to work. Other wise look for 1987 or newer

xm20k · Friday at 12:08 PM

My bet will be the TH350 will be a 6-inch tail from a car and not the 9" from a truck.

If it were me, I'd go 400 but I could care less about overdrive and changing the drive shaft yoke would be the excuse to change the U joints. Also more likely to be the 4" tail which would work with the yoke swap and shifting the crossmember back and not require messing with the drive shaft length.

Even if the 350 was a 9" that puts you 5/8" over the stock trans which could leave 1/8 of travel a bottoming out issue and lead to a broken tail housing.

jgloria223 · 2026-07-06T11:42:31-0400

Lots of great info from all the other guys. Honestly you need to add the question of how your going to drive the truck. Highway driving where the OD would be beneficial or short distance driving where the higher RPM's aren't an issue. You could also always change the rear end gearing to lower those RPM's at highway speeds if you go with a non-OD transmission. This definitely should be part of the equation. Good luck.

CalSgt · 2026-07-06T12:24:31-0400

It was the stock transmission and it lasted all these years why change?

I would build the transmission you have, maybe throw a mild TransGo shift Kit and corvette servo in while it's apart...

If the neighbor is selling the 700 for cheap it probably has whatever parts you need.

Transmission swap questions

86c10kid

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Ricko1966

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xm20k

Runs on 93 octane, caffeine, and spite

ali_c20

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Rickf

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86c10kid

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Ricko1966

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xm20k

Runs on 93 octane, caffeine, and spite

jgloria223

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CalSgt

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