a ka-t or sr would fit perfectly with the right upgrades.

the rb20 isn't a bad motor. but if you're shooting for mid 300s with good torque, i'd forego the money on trying to upgrade the rb20 and just get an rb25 or rb26, keeping the twins, and with intake, fmic, downpipe, and exhaust, you can probably get there without raising the boost very much. and of course water pump and oil seals are a must with any swap, just easier to do out of the car.