Trick Flow small block heads
The 273 small bore will eat a lot of the heads potential but there’s really no way you could come close to using it anyways you have to spin like 10,000 rpms to do so, 350 hp would be about 6000 rpm, 400 hp 7000 rpm, 450 hp 8000 rpm etc.. I usually think people over worry about velocity but in this case it would be definitely a concern, one good thing is you probably could make big power for a 273 with a relatively small duration cam with a .500 lift, solid would be a good choice.