what wheel/tire size for a 67 fastback

i have been looking at wheels and tires for my 67 fastback. hard decission...
for now im ordering new front and rear stock suspension that is 1" lower and im putting a 8.8" rearend in it. i have front spindle and brakes from a v8 car so its already 5-lug and im going to do a 5-lug conversion in the rear. im doing this just to get the car driveable (after 4 years of not being able to drive it).
future plans include an irs unit out of an 03 cobra, tcp or control freak type front suspension, and baer brakes. i will lower it more if it looks like it needs it.
im looking at 18" and 20" wheels that are made by coyswheel model c-5.
i think 18x8 is the widest for the front so can i fit an 18x9.5 or 20x8.5 in the rear, what backspacing would i need. or 18x9.5 in front and 20x10 in rear would be nice maybe. i can add fender flares for clearance on the outside but i dont want to mess with anything on the inside.

sorry its so long i tried to put all info you would need. ..thanks for the help..

i got an 8.8" rearend, motor, and T-5 out of a wrecked 92 gt. the motor got stroked to 331(built to be supercharged) and the T-5 and 8.8 are being rebuilt.
as far as the wheels being to big you are correct about the 20". but i know im not going smaller than an 18". the side of the fastbacks look thick to me, so i think an 18" fills the wheelwells up nicely if the car is lowered. you are also correct about the unsprung weight, and i did think about that but im not building a strictly drag car. at one point i was going to but the cops are terrible around here now, plus all the drivetrain i got was out of a car that was wrecked while dragracing.
so what about 18x8 in front with 4.5 backspacing and 18x9.5 in rear with 5 backspacing would that work???