The older chassis was built for an inline engine. There might be a lot of encroachment on the sides.

The Firepower Hemi is HUGE and weighs a ton. It would not be an easy fit into the engine compartment.

A Poly Plymouth engine (or Dodge pickup) is the same size as a LA but looks its age (older). They first appeared in something like 1956.
It would be easier to stick in your car, but would still be a chore.

Anything short of a turbocharger on the flathead 6 is still going to be slow. There were larger displacement Flathead 6s, as big as 265, but they are longer, IIRC. Still it might be an easier swap.

In reality, it is usually better to keep a car on the road. I have had too many projects drag on for years (one of my problems) and when it's sitting in the shop or under a tree waiting its turn you don't get a lot of enjoyment out of the car!
R.