Free US Mags...will they work for me?

I just picked up a set of US Mags off of Craigslist, for free. The price was right regardless of whether or not I can make this set of wheels work on my 74 Dart Sport.

All four wheels have a 3.5" backspace and the 5 on 4.5 bolt pattern.

Fronts are 15x7.5
Rears are 15x9

They were on an early 70's Dodge van.

I do plan to do a disc brake conversion so the front wheel bolt pattern isn't an issue. It may be in the rear.

I've been reading that a 7.5" rim is great for these cars with a 4" to 4.5" backspace. I haven't read anything about about a 9" rim with a 3.5" backspace in the rear.

So, with the disc brake swap up front and an axle swap in the rear, will these wheels fit my car? If so, any advice on tire size?

Thanks!