I know just what yer sayin.
wish you was a little closer, I have a real nice 88 wide box I would trade for the stepside.
how wide is you 8 3/4 housing flange to flange with out the backing plates?
you may have what you need already, flop it in there and measure up the right backset on rims and a 10" rim is no trouble..may not need to tub the bed.
the 67 polara/c-body 8 3/4 I have is 55" -/+ my stock truck 8 3/4 is 59" same as a 73 c-body.
check your housing measurement and post it up.
seems I ran a 10" rim with 3 3/4" backset with a 32x11.50x15 mud tire and it just did fit the stock stepside tubs/fenders.
so 10s should be ez to fit in there.