Quick question, I am interested in taking out the Th400 in my truck and replacing it with a 700r4 for the OD and lower 1st gear. Currently, there's a custom fabricated driveshaft that connects the rear of the Th400 to the input on the transfer case. If I swap in a 700r4, I would like to reuse the custom driveshaft off the back of the Th400. Would this work? Basically what I am asking is can I take the Th400 rear end (tailshaft?) and install it on the 700r4 without issues? I understand a complete disassembly is needed to remove a tailshaft, no problem as whatever tranny I end up with i'll be rebuilding it for GP's.

 

The 700R4 will likely last to at least the end of the driveway in your truck before grenading. Your only viable options are an external overdrive unit or a 4L80E (or the 6L80E or Allison trans if you really want to spend some money).

 

There seems to be a lot of debate on this issue of the 700r4 standing up in a 13,000lb truck. I contacted Tom at Alfa Heaven who specializes in doing power swaps in these old GMC's. To date he has tried a 366/Allison, 454/5spOD, 413/5spOD, 350/Th400, 8.2td/545 Allison, 478 diesel/5spOD, 6.2/Th400, 6.2/700r4, 355 stockcar, 671 detroit, 345 IH, gm 235, 4bt, etc.

His favorite swap was the 6.2 and 700r4 due to the availability, simplicity and fuel mileage. Compared to the Th400, the overdrive gave him an extra 2 mpg.
As for durability he has not had any issues at all even with the 1000 gal water tank on the bed full. I was told the 6.17 axle ratios are what keeps the load on the transmission light.

Myself, I won't be doing any heavy hauling or offroading. Just something to bring those RPM's down at 55mph. Currently my numbers are 2798rpm's at 55mph with the Th400, 455, 40" tires and 6.17 gears. I am betting it gets around 8mpg now.

 

I bet a 700r4 in good condition will handle a mild 455. Upgrade to a Corvette servo, install a shift kit with a .500 boost valve, and install the biggest trans cooler you can fit. If the trans needs rebuilt, install an upgraded 3-4 clutch pack with the other upgrades. Use a shift kit calibration for trucks or RVs, the idea is to firm up the shifts and get the line pressure up to a point that things wont slip under load. You don't want to knock the dash out of the truck with a hard hitting trans.