Need some help with wheels/tires for my 1966 C20 Fleetside

[Rat66]

Hey guys. It has been a minute since I have been in the forums. While trying to narrow down my front end bounce/shake that it gets at about 50MPH I started with checking the wheels/tires to see if they are out of balance or if there is some bulging going on. Since the truck has been in storage for a long time and I did not want to have to add it to my insurance just yet so I decided to take the wheels off and run them down to Discount Tire to have them checked and balanced.

I get there and we are about ready to unload them from the back of my truck when the guy decides to check the manufacture date on the tires. He said they were manufactured in 2009. Apparently their policy is that they cannot work on any tires that are 10 years old of more even though he says they look in good shape (because they have been storage inside for years). Anyway, I am looking for wheels/tires that will work on the C20. As I start looking into it I realize I need to figure out the correct offset, backspace, and tire diameter for my specific truck.

I understand what offset/backspace is and somewhat how it works. But I am having a hard time determining how I figure out the best setup for my truck. I would like the tires just tucked inside the wheel wells/fenders (I would like to lower it some). While I have the wheels and tire off the truck I measured from the wheel mounting surfaces on the axle/spindles to the inner edge of the fenders/wells. I came up with 5" in the rear and 7" in the front. From what I have been able to find the offset used on this truck is 0mm(zero) and a backspace of 4"-4.5". So according to my math, this would mean roughly a 7-8" wide wheel. In my head it seems like an 8" wide wheel with zero offset would work about perfect in the rear with the tire sticking out slightly from the wheel rim.

BUT what about the front? There is a 2" difference in the distance to the fender. What does everyone do to make the front and back look the same in relation to the fenders/wells? It would be nice if I could use the same offset/backspace/tires sizes for both but it does not look like I will be able to if I want to bring all tires just inside the fenders/wells.

My other question is about tire diameter. The tires that came on my truck when I bought it were 235/85-R16 (31.7" diameter). I have found documentation online that says this is roughly the correct diameter. But I have also read that the factory size tires for the 1966 C20 were 7.75-15, with a modern day equivalent 195/80r15 with a diameter of 27.3" (693 mm), and a tread width of 7.7" (195 mm). Does anyone know the correct factory size/diameter? I am hoping it is more toward 27".

I appreciate any guidance!
Shawn