i find it very helpful to use the 2.6 displacement rating for a 13b and a 3.9liter displacement for 20b's as a guide when picing specs for your single turbo.

playing with turbo calculators and looking at compression maps these liter values seem to show the more accurate numbers.

I find that rating 20b's at 3.9 seems to workout. you'll find that t72's will get about 600 at a bar and people have dyno'd and spooled about where the should (for a 3.9liter).
Try it and tell me what you think and it could solve the question....what size turbo? and how do i look at this compression map.

porting plays a big part too, we all know this.