wheel size/backspace question

[69Tom]

I'm am going to be converting my '70 C/10 from stock 6-lug drum brakes over to disc brakes. So far, I have some 2.5" drop spindles w/ 5 lug discs in front. I'm pretty certain there's 5 x 5 bolt pattern on the rotors (need to measure when I get home). Will go with a '72 12-bolt with 5 lug drums on the rear.

I am going to purchase some Wheel Vintiques 62 Series steel wheels. I want to get 15 x 8s all around so I can rotate my tires.

Questions:

1) Will I be ok with 15 x 8 in the front? I can't see an issue with rubbing with a 2.5" drop spindle, but thought I'd check.

2) What backspace should I get on these wheels? 4.25"?

Thanks