67 swb stepside wheel fitment

[HardKoreKustoms]

so i know this is been covered and beem lookign but cant find the answer that i need, i see alot on tubbing but since i am running open bed (floorless) all i need to do i trim the bed side up a little, but i am about to order a set on the mob steel wheels and wanted to run a 18x10 up front and a 20/12 in the rear can i do that or should i stick wit a 18x8 and 20x10, will i have any fitment problems and what backspace should i use to have the rear wheels tuck, also with the 18x10 i should still be able to turn stop to stop, the truck is going to be a custom frame that will sit about half inch from rocker when layed out, and about a 4 inch when aired up, if that helps, with full custom cantilevers.