uhmm, a zz4 or big block will NOT drop right in. It would work if you went through the hassle of swapping engine mounts and if you didn't place your motor mounts right, possibly trans mount and therefore the driveshaft.
The PONTIAC blocks are externally the same from the 326 up to the 455, don't confuse these with GM corporate blocks which are really just a small block Chevy. Buik, Olds, & Pontiac stopped using their own blocks around 1979ish give or take a couple/few years and some odd exceptions.
I do agree with rifraf in finding a 455 if you can, they would drop right in and give you more go. However there are readily available stroker kits to turn your 400 into a 455 and such engines are arguably stronger since they have less friction loss through the mains (smaller crank).