Enter the Hellephant.

Why? It’s a small block engine. It will easily fit in an A-body.

It is just a 6.2L small block stroked to 426 CID and a 3 liter blower on it. It is tame as a kitten during normal driving and a total beast when you kick it in the ***.

The biggest issue will be putting a drive train behind it that can handle what it can do.
Easy of installation and cost...
I working on my 66 dart now...not much room as you know you own one.