SIZE of Wheels .. help with info 66 GTO

[robbie9999]

well guys I am still going crazy trying to figure out rim and tire size to go with
leaning towards 18 (95%) sure of or maybe 19 ( my car in black with a 20 near it ) but 20's seem to big ?

for 18 what do I need for back spacing extra


here is a 67 gto with 20 inch wheels it does' nt look tooo bad but it is kinda getting 4x4 ish - as far as the chrome thing not a big fan

your thoughts on spacing or rim or what i need to do - I do not want to lower the car to much at all ?