Question on 350 Turbo to 400 Turbo swap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

phil g · 08-01-2018, 08:55 PM

I have a 77 short wheel base 2wd truck.
I want to replace the 350 turbo trans with a 400 turbo.
I know that I will have to change cross member location, mount, yoke,
maybe modify the driveshaft length and change the kick down setup - my question is will the 400 fit without interference with the floor pan. Two of the shops in the region that do lots of square body vehicles claim that the 400 is physically too large to fit a "low hump" floor board and I will have to modify the floor to get the larger transmission to fit. Not a big deal, I just don't want to cut up my floor. I think that they are full of it and the 400 will fit without problems.
I want a 400 because I want this to be a backup tow vehicle for a small tandem trailer.
I would like feedback from someone that has actually done this swap and not just read about it on the internet. Thanks for your input

69heavychevy · 08-01-2018, 09:15 PM

My buddy has a low hump cab with a th400 underneath and he didn't have any clearance issues.

sweetk30 · 08-01-2018, 10:08 PM

should fit just fine .

and fyi th400 eats 20-25 hp over a th350 to spin it .

and a good quality parts built th350 will hold up just fine . and i am not talking crazy money or parts in it .

th400 = vac line to mod on pass side . electric kick down switch for factory gas pedal bracket in cab . shorten drive shaft if its not a factory baloon shaft you then need new one . move crossmember back prob 1 set of holes and re drill 2 in frame .

otherwise should be basic bolt in . maybe check flex plate bolt pattern on your engine to make sure th400 lugs line up before you install and find out it wont .

**kwmech** · 08-02-2018, 12:38 PM

If the 400 doesn't fit the floor pan when installed, it is time to replace the body mounts. It'll clear. My 69 1 ton has a t400 in it with low hump---factory

cadillac_al · 08-02-2018, 03:34 PM

It fits, been there done that. TH400's even fit in Camaros. They aren't that much bigger than a th350.

Honest Bob · 08-02-2018, 03:40 PM

My 75 C30 is a low hump with a th400.

donw1986 · 08-03-2018, 03:11 AM

I have an 86 that came with a 700r 4 and i put a 400 in it no problem i beleive mine is a low hump

**Warrens69GMC** · 08-03-2018, 09:40 AM

I just parted a 79 GMC c1500 low hump with a factory 350/th400 combo. Was a heavy half ton truck. It will fit without problems on SBC

08-01-2018, 08:55 PM	#1
phil g Registered User Join Date: Mar 2007 Location: Bristol TN Posts: 18	Question on 350 Turbo to 400 Turbo swap I have a 77 short wheel base 2wd truck. I want to replace the 350 turbo trans with a 400 turbo. I know that I will have to change cross member location, mount, yoke, maybe modify the driveshaft length and change the kick down setup - my question is will the 400 fit without interference with the floor pan. Two of the shops in the region that do lots of square body vehicles claim that the 400 is physically too large to fit a "low hump" floor board and I will have to modify the floor to get the larger transmission to fit. Not a big deal, I just don't want to cut up my floor. I think that they are full of it and the 400 will fit without problems. I want a 400 because I want this to be a backup tow vehicle for a small tandem trailer. I would like feedback from someone that has actually done this swap and not just read about it on the internet. Thanks for your input

08-01-2018, 09:15 PM	#2
69heavychevy Registered User Join Date: Jan 2007 Location: Garland,TX Posts: 431	Re: Question on 350 Turbo to 400 Turbo swap My buddy has a low hump cab with a th400 underneath and he didn't have any clearance issues. __________________ 80 C10 SWB Step Side - 350 w/ vortec heads, TH350, 12-bolt 3.73

08-01-2018, 10:08 PM	#3
sweetk30 Registered User Join Date: May 2012 Location: horseheads,ny rust belt Posts: 2,794	Re: Question on 350 Turbo to 400 Turbo swap should fit just fine . and fyi th400 eats 20-25 hp over a th350 to spin it . and a good quality parts built th350 will hold up just fine . and i am not talking crazy money or parts in it . th400 = vac line to mod on pass side . electric kick down switch for factory gas pedal bracket in cab . shorten drive shaft if its not a factory baloon shaft you then need new one . move crossmember back prob 1 set of holes and re drill 2 in frame . otherwise should be basic bolt in . maybe check flex plate bolt pattern on your engine to make sure th400 lugs line up before you install and find out it wont . __________________ 77-k30lb BIG truck build . 87-k30lb budget beater build . 85-k30lb the plow machine build . 85-c10sb summer fun toy . ----------------------------- HOLLEY SNIPER efi = worst case of p.i.t.a i ever had . EDELBROCK pro flo 4 = best deal going so far . love my setup works great.

08-02-2018, 12:38 PM	#4
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,650	Re: Question on 350 Turbo to 400 Turbo swap If the 400 doesn't fit the floor pan when installed, it is time to replace the body mounts. It'll clear. My 69 1 ton has a t400 in it with low hump---factory

08-02-2018, 03:34 PM	#5
cadillac_al Registered User Join Date: Oct 2013 Location: Maine Posts: 2,376	Re: Question on 350 Turbo to 400 Turbo swap It fits, been there done that. TH400's even fit in Camaros. They aren't that much bigger than a th350. __________________ 76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10

08-02-2018, 03:40 PM	#6
Honest Bob Registered User Join Date: Jan 2017 Location: Easton PA Posts: 219	Re: Question on 350 Turbo to 400 Turbo swap My 75 C30 is a low hump with a th400.

08-03-2018, 03:11 AM	#7
donw1986 Registered User Join Date: Oct 2012 Location: Santa Clarita CA Posts: 80	Re: Question on 350 Turbo to 400 Turbo swap I have an 86 that came with a 700r 4 and i put a 400 in it no problem i beleive mine is a low hump

08-03-2018, 09:40 AM	#8
Warrens69GMC Registered User Join Date: Jan 2003 Location: Safford,AZ Posts: 3,625	Re: Question on 350 Turbo to 400 Turbo swap I just parted a 79 GMC c1500 low hump with a factory 350/th400 combo. Was a heavy half ton truck. It will fit without problems on SBC __________________ 1969 307, t350 1950 Chevy Wagon 1978 Big 10 1967 C10, 250,3-OTT



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