After going through all the comments, I’m starting to lean towards swapping in the 400.
I kinda wanted the 318 build to make more sense since it would be logistically easier. But the more I think about it, the big block makes more sense in the context I would use my car. I will drive it a lot and would spend 99% of the time being driven like a granny.
The big block has a bigger bore and bigger valves. With some porting my cast iron heads should beat the small block edelbrocks easily up to .400 lift. I can have the heads milled to bring compression up to 8.5 and run a small cam on a 108 LSA like an Isky 262, and the big block will make more power and torque than the higher compression 318. I might be a able to run it on crappy 87 octane even. And if in the future I find myself financially able to build a proper big block stroker, the car is already set up for it.
I can always build a small block screamer later for another car. And regarding building a 360LA or Magnum instead, I have thought of that too. 360 LAs are hard to come across here in PR, but there’s plenty of junk 1990s Rams and Durangos I could pull an engine and trans from. It would be an easier swap vs swapping in a big block, and cheaper. But again, a big block would beat it in airflow through the heads and torque.