Wheel/Tire help

VNTW51577658865 Torq Thrust II 17x7 & 18x8 5x4.5 Package
I want to put new wheels and tires on my 71 Demon, it now has front disc brakes with bbp and i am going to replace the old axles with bbp and drum setup 8.75 rear end.
My question is.. What backspacing and wheel offset do i need.
I've been reading and looking at everyones rides here on the forum and really like the looks of the Torque Thrust 2's. I'm thinking 17" front and 18" rear.
I have a link to what i have picked out for reference. Just not sure on backspacing and offset. Tire size is still up in the air also.

Thouhgts?