If you're not an experienced autocrosser, you're taking the notion of "building an autocross car" far too seriously. The driver is the biggest part of the equation. Doesn't matter what you do with the engine or suspension until you've gotten past that hurdle.

But, in general, you want to minimize shifting in autocross. So, you want an engine with a very wide powerband that has some "dig" from low revs. Something that you can get a good launch in first, shift to 2nd and leave it there whether the course slows down to 25mph or speeds up to 65.

How you acheive that kind of powerband depends on your specific engine. But, really... as mentioned above, I wouldn't stress about it. Just don't build a "peaky" engine that doesn't make any low-mid range power and you'll be happy. (in other words, build a typical factory-spec engine)

Reliability and consistency should be your goals for your car while you're learning.