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

Just asking
If you can physically fit a slanty in there....... what about a smallblock? Then you won't need a turbo, cuz even a 318/5.2 will do 250/320 without even breathing hard. And the power-delivery will be freight-train flat and start right off idle.
There is an AX-15 5 speed that will almost bolt right onto the SBM.It comes in two flavors, the close ratio having excellent progressively shorter splits, and a nice overdrive ratio.
If you go late Magnum, it is already all set up for EFI and the AC compressor may already be on it. And it is rated 230hpNET, with lots and lots of Torque. IDK the T-spec but I bet it's close to 400NET.
The weight difference from slanty to SBM is very small. By the time you put the turbo kit on the 6 the weight on the front wheels will be very similar to the SBM.In fact, the SBM may be better,cuz the weight will be further back.
But before you get started, look at the steering very closely.

If you can fit a 318/5.2 in there, well, then you can also fit any other SBM based engine in there..... all the way to 416. and a tad more,lol.
But of course that brings more issues, like broken rear-ends, rear springs, and gearboxs.