Check your front subframe to see if you don't already have two sets of holes in it. Lots of GM cars from the mid-60s all the way to the late 70s have two mounting locations for the crossmember.
The one closer to the engine is to locate manual trannies, PowerGlides, and THM 350s. The one further to the rear is for the THM 400s. Some '68 and '69, and possibly even a few '70 Novas came from the factory with big blocks and 400 THMs. It's also possible to use a factory driveshaft from a big block/THM 400 Nova for your application; IF you can find one. It would save you some expensive fabrication work.

I agree with the other guy about using the THM 350. They are plenty tough enough to handle any smallblock Chevy with no more than a good (TransGo, B&M, etc.) shift kit added. And, a THM 350 weighs about 50 lbs. or so less that a THM 400, and that's equivalent to installing aluminum heads.

The only reason I could justify a 350 to 400 swap myself would be either because I switched to a nasty big block, or because I preferred the superior shift quality of a 400. Ain't nothin' shifts like a 400 Turbo in a Buick Electra; silky, but firm Man, and you gotta love that!.

Randy