I don't think you need anything super special to make a 904 happy behind a turbo'd slant in that power range (assuming torque is around similar levels). The key is quick shifting valve body with higher line pressures to hold things. My tranny runs 190 or 210 (can't recall) with a 4.2 lever and I put around 550 ponies though it and only slightly less torque. Mine is a 999 case and internals because the case is supposedly stronger than the early 904's, and it had a low gear set (2.74 instead of 2.54). For whta you're building, you don't need a limits pushing setup, or even the setup that I run. A good basic build with an eye to quality clutches bands and good Valvebody selection and you'll not have a problem.

BTW, I love the idea of a turbo slant

Greg