1960 step side backspacing

[Samz]

I have a '60 short bed step side Chevy. I just installed 4" drop coils in the back and put my bed back on (I've been painting my truck, so the bed was removed) and now the tires are extremely close to the outer fender....about 1/2 inch from rubbing. I have some 10x15 rims (unknown backspacing/offset). My tires are 235/70 15's in the rear, and 215/70 15's in the front. I like the stance of my truck with the 4" drop coils, but obviously I'll have to correct the fender/tire issue asap. Does anyone know what backspace/offset I need to fit my rear tires? I really don't want to replace the rear coils with a 2" drop coils because I like the way it sits with the 4" drop coils. But, it swapping out to the taller coils is best/safest, then of course I'll go that route. The front has the torsion bars cranked down (previous owner did that). I'm waiting on my 2 1/2 drop spindles to show up any day now.

The Blue pic is with the 4" drop coils. The primer pic is they way it was before the new coils were installed. The previous owner had the stock rear coils clamped down to make them sit shorter. I've been going through this truck inside and out. I have pretty much all the new parts sitting in the garage waiting to get installed. I should have all the paint completed by tomorrow, so I can start putting it back together and get started on the interior and the Vintageair installed. Thanks in advance....