Turbo recommendations

2/3rds would give you instant response and low end torque but no top end. If you go too small it will be hard to control boost, the waste gate may not be able to keep up. Plus your intake temps will be high from a turbo which is too small.

The 2.3 turbo ford guys get used Holset HX35 turbos off of diesel trucks like the Cummins 5.9. They are a little on the large size for a 2.3 gas motor but should work on a slant six. The HX40 is another option it is the next size up. A rule of thumb for using a diesel turbo on a gas car you divide by 1.9. 5.9/1.9 = 3.1 liters gas. Course for turbos you go a little larger for high rpm horsepower and a little smaller for less turbo lag.

It takes a 2.3 turbo motor about 3000 rpm to fully spool an HX35 so there is a bit of lag. Having a turbo a little too large is not a problem as far as horsepower goes but there will be more turbo lag. The 2.3 guys get up to 400hp from a HX35.