When I got my 72 Dart it had a slant six.

The most cost effective thing I did to improve it's power was to put a 360 in. But this was back when you could buy a good running donor for $100 and V8 A body K members could be bought cheap int the yards. I had less than a grand into the swap including a new dual exhaust and radiator. The wiring harness needed minor mods to work, the donor supplied the engine and trans. Had I not already updated to 11.75" discs it could have provided that as well as it is I ended up swapping those onto my 64 300.

Later sold the slant for $250, including installation.

Today, I doubt you could replicate it for anywhere near that.

Depending on fab skills you could put a turbo on, EFI it, mill the head for compression. I am building a 218 flathead doing similar. But the head alone exceeded the cost of the 360 swap I mentioned. Just remember, like the flatty, the slant doesn't really like higher rpm, the piston speed gets stupid high. So do not build for HP, build for torque. A small turbo is probably the best there and no need to mill the head that route.