[MrPbody]

For the most power, always choose the larger engine, especially for street use. "There's no subst\itute for cubic inches" is how the saying goes.
However, the 400 Chevy, while powerful and a good stocker, requires significant upgrades to the block to withstand the rigors of high performance street engines. The block is the weak link. This is the reason so many build 383s ("strokers"). A 350 block is much tougher than the 400.
The rusty 327 is probably not a good choice, as it's going to need virtually everything in a rebuild (possibly including some sleeves). The bang-for-the-buck with 327 isn't what it used to be. 350s are thre current Chevy of choice, for the majority.