oleary

I an changing out my 350 Rocket to a 455 on my 70 cutlass. Will I need to change the rad ? Fusick also sees a motor mount conversion kit , is this kit necessary ? Will the drive shaft be ok...350TH to 400

svnt442

1. Maybe. It will all depend on what is done to the 455 and how hard you plan to run it.

2. Nope. Use the mounts for the 350 and be done with it.

3. Nope. Not only are the 350 and 400 different lengths, but there are more than one length tail shaft on either trans. Then there is the yoke needed being larger for the 400.

4. (you missed one) The parking brake cables will need to be changed due to the position of the transmission cross-member being further back with the 400 vs the 350.

jensenracing77

another thing to watch out for when going from TH350 to TH 400 is the exhaust. i did this to a car and found that when i tried to move the crossmember back the the 400 location it was hitting the exhaust. the exhaust shop bent the exhaust as soon as it was behind the crosmember and had no room to move it back.

oleary

perfect thanks...

What are the options on the drive shaft ?

svnt442

Best bet is to have one made up. Messing with a 40 year old piece of metal and trying to have it shortened it just asking for trouble.

rustyroger

Don't forget it a bit heavier as well, the front springs might need upgrading and tire pressures might need to go up a few psi.
Roger.

joe_padavano

The difference in weight is about 50 lbs. You'll be hard pressed to see the difference in ride height.

atkinsom

Tire pressures were mentioned here so figure I would ask...for cruising and not racing what kind of tire pressures are recommended for a SBO and BBO on a cutlass

timothypaul

Hmm - I did this swap and had no issues with the driveshaft. Also, you can use the 350 motor mounts, I did, but the engine will sit a few inches higher. If you want to use 455 mounts you need to change the mounts on the frame as well. It depends on your intake manifold and air cleaner height.

Don't forget the kickdown will change, I have had trouble with that. The T400 kickdown is a switch attached to the gas pedal whereas the T350 is a cable coming off the carb. For some reason, I can not get the switch to sit properly with the gas pedal, the throttle cable is in the way.

Also, be careful which headers you use if you decided to go that route. I had to move my brake proportioning valve.

Is the 70 different from a 71, I did not have any crossmember issues on my 71?

joe_padavano

The TH400 is longer than the TH350. The driveshaft needs to be shortened AND the front yoke is larger. It is not physically possible to use a TH350 driveshaft on a TH400. Better check your memory.

And yet again, the 455 sits "higher" than the 350 because it IS both taller and wider - hence the term "big block". So long as the rubber motor mounts and metal frame mounts are matched on an A-body, ANY 1964-1990 Olds V8 will sit with the crank centerline in EXACTLY the same place. It doesn't matter if you use the so called "small block" (2261) or "big block" (2328) rubber mounts so long as they match the frame mounts. You CANNOT put the engine any lower with any combination of mounts because the oil pan will hit the crossmember.

1968-72 crossmembers are the same, and again, the TH400 is longer than the TH350, requiring the crossmember to move rearward. If you had no crossmember or driveshaft issues, you DIDN'T swap in a TH400.

455man

I kept the 3 core radiator in mine when I put the 455 in. It ran about 210 on the highway on a 100degree day. Most of the time it was fine. I'll probably use an aluminum radiator this time.

72 w29 all green

Also if your car is "auto on the floor", then keep in mind the Turbo 400 requires a different bracket than the Turbo 350 to mount the floor shifter cable.

Stattiolds69

Hey Guys,
If I may chime in, IMO if you're not gonna be using sticky tires and strictly drag racing your olds, i'd stick with the th350 and save the hassle. I put a stout 455 behind a well built th350 and ran it quite hard for a few years with no problem at all. You'll save about 10hp too with the th350 using less power to turn it as opposed to the th400, plus the expense and hassle of the change over in general. I've done the swap before myself but it was unnecessary, at least for me. I guess it depends what you'll be using it for primarily ?

CJG

I see this is an old thread but I have a couple questions regarding the same swap. I'll be swapping in a very mild built 455- .30 over with a 278 cam, re worked port matched stock J heads with stainless valves, hooker comp headers. I was thinking of keeping the TH 350 trans and maybe putting a shift kit in it and going to a 3.55 or 3.73 posi rear diff. Any suggestions on if that sounds ok or anything I should do differently?

455man

It's been done but some dont like a TH350 behind a 455.

oldcutlass

If you beat it like rented mule the stock TH350 will not live.

cjsdad

I thought that was a government mule? Oh well, if you beat it like a mule, it will have issues