THM 400 to 700 R4 conversion

[Rnd]

I just finished up the paint and cosmetic work on my 1977 GMC 20 Crew Cab (see pics thread).
Took it out for a 60-70 mile ride and no surprise, the fuel mileage sucked and it turned some relatively high rpm's on the thru-way.

I'm thinking of swapping out the 400 trans. and installing a 700 R4.
I see Monster Trans company has a conversion "kit". Has anybody done this?? If so what should I be looking for in the way of unforeseen expenses??

Tech. info:
454 stock (for the time being)
400 trans
4:10 floating rear axle
2 WD

Any thoughts or input from anybody that has first hand experience would be greatly appreciated.

Thank you,

[jimbonice]

I have a 700r4 in my C30. Plan on doing a good build with a decent torque convertor, corvette servo, good clutches, etc. Other than changing the yoke and fabbing a driveshaft, I had no great expenses.

[midnight rambler]

Ive been thinking of doing this swap as well i currently have a 4 speed. I will most likley pay a transmission shop to do it for me as i dont even have the time to drive the truck. I will keep an eye on your thread.
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[bluex]

Originally Posted by jimbonice

I have a 700r4 in my C30. Plan on doing a good build with a decent torque convertor, corvette servo, good clutches, etc. Other than changing the yoke and fabbing a driveshaft, I had no great expenses.

[IMG]https://lh3.googleusercontent.com/-oAyZICxD69E/UenumZBvsJI/AAAAAAAAElo/8Cc0fNmH4e8/w800-h600-no/IMG_20130714_161343[IMG]

WOW! Beautiful Dually!


I swapped over mine to a 700 and I dont know what all is in monsters kit and what it costs but I got my TV cable stuff from Bowtie overdrives and I hightly recommend them, the trans and converter from a local shop and the lockup stuff I got from a friend that worked at the dealer and wired it up myself. I cant remember what it all cost but it wasnt to difficult to do, just slide the crossmember back and drill new holes. I had to have my driveshaft shortened as well but I also convereted my truck to a short bed at the same time.

[Jonboy]

Originally Posted by jimbonice

I have a 700r4 in my C30. Plan on doing a good build with a decent torque convertor, corvette servo, good clutches, etc. Other than changing the yoke and fabbing a driveshaft, I had no great expenses.

Do you remember how much had to come off of the driveshaft? I am getting ready to do something similar, but wanted to make sure the driveshaft would work. (5.3 and 4L60E swap).

[SCOTI]

Originally Posted by Jonboy

Do you remember how much had to come off of the driveshaft? I am getting ready to do something similar, but wanted to make sure the driveshaft would work. (5.3 and 4L60E swap).

Jon... If I recall, you'll also be swapping in a 12bolt?? The lengths will be different because the differentials are diff sizes too.

[mrolds88]

Instead of going 700r4 in a heavy vehicle like that crew cab, why not do a 4L80E? Easiest way to accomplish is to use the trans computer out of a 91-93 diesel truck. That way you dont have to get all the high end parts for the 700.

[Jonboy]

Originally Posted by SCOTI

Jon... If I recall, you'll also be swapping in a 12bolt?? The lengths will be different because the differentials are diff sizes too.

This is true. From what I have read, there is only 1" difference between the 6" tailshaft TH400 and a 4L60E. I just wanted to make sure I didn't need to get the front half of the driveshaft shortened at the same time I do the rear. I will be using the Dirty Dingo sliding mounts, so it may be a non issue.