Chevy Big Block

I have a 64 C10 that had a 350 engine with 350 transmission, I am Switching to a 400 turbo transmission and a 454 engine.

Do I need to change the motor mounts and perches or will the 454 bolt up to the existing mounts/perches?

I've never done that swap, but I recall the 350 engine mounts will bolt up to the 454.

I also think the 400 trans has the same mount location as the 350, but I am not sure. However, a minor relocation for the trans crossmember is very easy. Just drill new bolt holes in the frame where required.

Every 400 trans I have used requires an electric kick-down switch. Without it, the transmission will not kick down to second for highway speed passing. The THM 350 uses a throttle position cable for this, which the 400 does not have.
So you may have to use a kick-down switch attached to the gas pedal or get used to shifting manually for a kick-down.

I used to have a 1968 Impala with a small block, I was told that the big block 427 was a drop in replacement and would not require any modifications other than stiffer springs for the front end. So your 454 shouldn't require any modifications either.

Yes the TH-400 has an electric kick-down for the Drive to 2nd gear change, the Quadrojet carb had a push button on it that was activated when the throttle was pulled back far enough, it worked very well.

Yes the TH-400 has an electric kick-down for the Drive to 2nd gear change, the Quadrojet carb had a push button on it that was activated when the throttle was pulled back far enough, it worked very well.

That sounds like a good place for it. My THM 400 vehicles ('81 Fleetwood, and I think, my '69 GTO had gas pedal switches.
But my old '77 Jeep had a carb-mounted switch, I think.

The engine and motor mounts will fit just fine, (although some 4x4 trucks require you to swap the frame-mounted M.M. brackets from L to R). And a TH-400 trans should not even require you to relocate the cross-member. What I DO think will be a minor issue, is the driveshaft. if you use the same tailshaft-length on the 400 as the 350, the shaft length will match, (If you use a long -tailshaft T-400, you'll need to have your driveshaft shortened). BUT, I am pretty certain the driveshaft yoke is different on a 400 vs 350. (The number of splines on the trans tailshaft must equal the spline count of the D/S yoke). Not a big deal, though. Most any driveline shop can sell you a T-400 yoke for your driveshaft. It may require different U-Joints, but you'll be wanting to replace those old, tired ones anyway, or that will be the most likely item to fail soon, once you put some big rat-motor twist on them! As for the kick-down switch - why worry? With Big-Block torque, that beast won't need to downshift at highway speed anyway! :) (My Chevelle & Monte are both BB/T-400 cars, and both have vacuum modulators on them, which control the upshift points. Neither uses a kickdown switch, the wires have been clipped off the trans switches. No problem at all, if I really lug the engine speed down low in 3rd gear, and need to accelerate, I just downshift it myself, but 99% of the time I don't need to, the big-block takes care of things just fine). And trucks generally have lower rear gear ratios than cars, which will further reduce the need to downshift... JMHO. Good luck, let us know how the project goes!

Yes Ol'Jim the couplers for the TH-350 and TH-400 are different, they are not that hard to swap though.

And yes the tail shaft length of the tranny may be an issue, I would measure the tail shaft housings of the TH-350 that you have and the TH-400 that you are getting and then decide if you want to have the transmission modified or the drive line modified.

If (and i'm saying IF) you want to have the TH-400 rebuilt before you bolt it in now would be a good time to have the tail shaft housing replaced also to one that will work with your truck (if that even ends up being an issue).