340 intake on a 318?

Why not just get the correct one so you don't have to guess whether it will work correctly or not? Most times you will have no problem with the differences in port size, but, I have personally seen a couple where it caused a bog. These were on the earlier heads though, so, that might not be a problem with the 302's since their intake ports are kind of in between the old 920/675 castings and the 694/915/596/ castings. Cam choice and rear end gearing can play a part here too.