Considering a Slant 6 swap into a Mitsubishi Starion...

A slant is much longer than the small-block V-8. They offset the water pump and set it into the block, but even then one can't quite fit a clutch-fan, even a thin one from a Jaguar or M-B. If your heart is set on a straight six, the Jeep 4.0L might be a better choice since it lived into the MPFI days (2000's). Only a few people have done MPFI in a slant and very custom. The only way to get EFI w/ bought parts is a 4 bbl intake (Clifford or Offenhauser), plus TBI (many 4 bbl carb replacements). I haven't heard of factory turbo for a 4.0L, so would have to be custom like a slant. I recall the exhaust is on the same side as intake, which makes plumbing easier (unless adding an inter-cooler). Some people love a straight six, like BMW owners who whined thru the years it changed to a V-8 (then brought the six back). But, BMW parts are $$$. If you want a V-6, consider the Mopar 3.8L. It has a better 60 deg V angle than the earlier Magnum V-6. It was used in Jeeps, so there are RWD transmissions and motor mounts, plus Jeep guys degrade it as "minivan engine", which is stupid but may have led some to remove them for a 4.0L.