Looking for tire size recommendations (and pics if ya got 'em)

[brossow]

Just ordered the Boss 338 wheels for my '72 shortbed stepside but am at a disadvantage when it comes to actually sizing up the proper tires because the chassis is in my garage but the bed is in another town in my buddy's body shop and therefore I can't measure as I would like. Hoping someone else with a similar stepside can tell me what they're running so I can get tires ordered before the truck is actually back together. The truck will have about a 4/6 static drop with leaf springs out back.

Here are the wheel specs:

Front: 20x8.5, 20 mm offset, 5.239" backspacing
Rear: 20x10, 20 mm offset, 6.225" backspacing

Rear
I'm planning to run a 1" adapter (spacer) on the rear, effectively making the backspacing 5.225".

I want the biggest tire I can fit out back without even a chance of rubbing. I had been thinking 295/35 or 295/40 but there don't seem to be many options in those sizes, and price is important. At about 3/4" narrower, would a 275/35 be too narrow on this rim?

Front
95% sure I'm going 245/40 up front. Any reason not to?

Thanks in advance for any advice!

Brent