What is the largest wheel/tire I can go with a 4/6 drop

[62ChevyStep]

I have a 62 Chevy SWB Step, looking to do a 4/6 drop using coils and spindels. Front end has been converted to coils already. five lug all the way around, want to go with Boyd coddington Smoothie II's.

I have a 71/72 12 bolt 3.73 in the rear so it is 3/4" wider (each side) to start.

what would work best 17", 18", or 20"'s using no modifications?

what should go on front (ie: 18x8.5-5"BS) with what size tire?

and on the back?

Sorry for all the questions I am new to this wheel tire game and don't want to spend cash and time expirementing.... Thought I would ask people that would know.