Bore size is one of the ultimate main limiting factor to hp but for most were not making enough hp to overly worry about it, most aren't even pushing what a 318 bore is capable of and really a 360 bore is less than 3% bigger than a 318 bore and a 340 bore is slightly more than 3% compared to a 318, even a 273 bore should be able to handle 400 or so hp without crazy work. For most bore size ain't a huge issue, making power at an reasonable rpm for that person is probably a more pressing issue for most which usually mean a higher power to cid ratio aka no replacement for displacement.