850 or 950 Carb

If a performance engine cannot eclipse 90% efficient, you're correct. Except most can. The longer strokes tend to easilly reach 112% at peak torque, and will carry that well past HP peak. So you are correct, but the 383 is a short stroke. You could also use the percentage of stroke increase and multiply the carb choice the general "rules" say. 750x 120%=900.