Going to 7/16ths means you'll have to drill out the holes even larger for the helicoils...this is bad!
This is bad because there isn't much aluminum around each hole as it is...this means less meat...the aluminum is already soft from annealing so now you'll be removing more material....this can promote cracking, further striping, coils too just pull out over time (you'll notice the nuts won't stay tight after you put some miles on)...it's just bad!
I recently rebuilt my head...my head wasn't annealed (or atleast not as bad as I've seen)...but what "I" did was, helicoiled for a smaller stud than stock "3/8-16"....this meant I didn't have too drill out much aluminum at all!
I'm not arguing as the only people who would argue this simply aren't that smart when it comes to this type stuff....I machine for a living so I just kinda see this stuff as basic...although I consulted ij about doing this first he too agreed with my idea and it wasn't new info to him either as he is a machinist also with years more experience than I!