Rpm is gonna be your enemy, 300 hp is gonna be about 5500 rpm 400 hp will be around 7000 rpm other then that should similar parts to do the job, probably don't want to go much bigger than a 1.88 valve.
If your gonna build a 273 or even a 318 you can't be shy about using deep gears, high rpm small engines need a lot of gearing if that is a problem you'd be better off with a bigger engine. Plus you need higher stall speed.
7000 rpm x 28" tires / 110 mph x 336 = 5.30 rear gear
5000 rpm x 28" tires / 110 mph x 336 = 3.79 rear gear
The usual set of 3.55's (4686 rpm @ 110 mph) is generally closer to ideal for a larger engine than a smaller one.