Are their big dollar differences between the engine sizes and what it takes to put them in? Jim
Install is pretty much synonymous between them, it is primarily a difference in funding the initial power package. You can get a good rebuild-worthy non-Eagle 5.7L for $900-$1,500. With coils, some accessories, fuel rail, intake, etc. A decent mileage 6.1L will run you around $5,000-$6,000, a new 6.4L will push $8,000 for the long block.
My personal path to success is to start with a basic 5.7L. I can easily make ~320whp with it, the initial cost is much lower and will help me actually get "the swap" accomplished. Once I get things settled, the bugs out of it, the transmission setup..then I can work on a larger engine if I please. However, I can also add a small amount of boost to the 5.7L and run a easy 450-500whp without much trouble. However, going from 240whp [318] to around 320whp and from a 904 to a 6-speed manual..I'll be giddy enough to not notice the "lack" in power. The car should be good for an easy 12-second 1/4 with a double-overdrive and net 20mpg or so on the highway.
It's all in what you want to do. Sure, I'd love to have a 6.4L in my car, however I much rather
have a 5.7L than nothing..or an old LA small block. Affording a high-end engine right off the bat would push my project budget too far and it would either never get started, or never get finished.