stupid step side bed ?'s

[blazentrout]

Newbe here with a couple of?. I just got a '64 gmc short stepside that is/isnot a basket case(missing alot of bolts,alabama hillbilly truck).so i want to do some stuff to the bed.currently there is no floor,just a couple of sign posts holding up the bed(will be steel when done). i am trying to figure out about where it should sit hieght wise and how far back from the cab. the general direction that i am going is a early '70's bar hopper style (slots,peace sign, 4on the floor).
my rear fenders are repairable, but need to be widened(i'll do the work myself)to work with the tire combo but i need to what will be more cost efective.
i am not sure if a 295/50/15 on a 10" rim will work with 3" wider glass fenders and a '77 12bolt rear end. Thanks for the help,Trout