you really need nowhere near 300 hp for auto cross, plus with a supercharger setup on any of these combos would make it rather nose heavy. I would go for a high compression b16. It will be light, dependable, cheap and easy to work on, and as long as the compression still works with pump gas, youll have a car you dont have to put much money in.
A b16 with a gsr tranny aand type r casing, high comp. pistons, a ported head, maybe a sleeved block, upgraded internals, and you should be good to go with a decent weight distribution...