wheel size help

[CG]

i went to order some wheels for my burb today. im going 15x8 all corners. as far as tire size im not totally sure but nothing huge and fronts a little bit smaller than rear. basically im going for a sort of 60's look with some 3/4 in. white walls. its going to have a 4/6 drop when done. i was going to get 4.25 backspacing per advice from a board member but today i found out the wheel company doesnt offer it. only a 4 or a 5 is offered. he said maybe i could go with a 15x7 wheel for centering purposes. his 15x7 have exact same dish as the 15x8 any advice out there?