For more power I would install a mild 401, (or at least a 360 AMC), it won't cost much more than going through the 304, especially if you build it for power, and as stated, it will bolt right in! The mild 401 would be more reliable and make more bottom end torque than a built 304 with the same horsepower. Note that the 401 is balanced differently than the 304, so be sure the flywheel is made for a 401, or have the 304 unit rebalanced. The vibration damper is different, too.

If you want to go for max power, the 401 would be way ahead of the 304 (and 360) as a starting point. The stock crank and connecting rods are forged, instead of cast, and the block is alot beefier.

Here's a good source for AMC parts:
http://www.bulltear.com/index.php

Here's a good AMC related forum:
http://amccars.net/cgi/yabb2/YaBB.pl