That is all going to come down to this $$$$ or this $$$$
The fixed hemi is going to probably run a easy 5k, between motor, mounts, headers, oil pan and all the wiring and computer nonsense if you keep the f.i., if you don't keep the f.i. i doubt you'll save much.
To make the 18 into fun is going to cost about as much, since you are going to need heads, roller cam, crank and pistons, rods are fine, but intake and headers also and carb.
OR
You can look for someone selling a built LA motor or even a crate motor ready to go