[RarlyL8]

320HP at the crank is way too conservative if adding a turbo and EFI, even if using a 915.
The 915 can live just fine at 300HP at the wheels. With mods it can live at 350whp as long as you let the turbo do the work, no drag starts or speed shifting. A stock 915 can handle 320CHP no problem. Reinforcement for longevity and added strength would be a reinforced side cover and larger bearing retainer. LSD helps evenly distribute the load for even more strength. I ran that combo in front of a 3.3L 930 engine in an SC for 15 years that saw variations in output from 300whp to 350whp depending on what we were testing at the time.

Least expensive and problematic turbo install would be to use OE 930 parts. Install becomes plug and play with only the intercooler being custom at that point. This is assuming a standard flange pattern turbo is used. OE "Euro" heat exchangers and J-pipe would help keep the power level down closer to your target as well. Protomotive does a side mount turbo so they have those parts if going that route.