Rim size suggestions please?

[GMJimmy]

I have a 68 SWB FS with 2-1/2" drop spindles and disc brakes and the rear axle is out of a 1972 C10 with drum brakes. It presently has 15X8" 5X5 bolt steel Chevy rally wheels on it front and back. Tires on the front are 245-60-15 and on the back are 275-60-15.

My rally's have a 4.25" back space so I was hoping to find something similar. I'm looking at some Rocket Igniter Gray wheels for it but I can't get the same back spacing on a 15X8" so I'm looking at the 15x8 with a 3.75" back space for the rear and a 15X7" with a 4.25" back space for the front. I was going to put the 15X8's front and back but am afraid the 3.75" back space will cause front tire clearance issues???

I have searched quite a bit here but can't find any specific do's and don't for these trucks in regards to wheel size. Any help would be appreciated.
Thanks
Jim