68 mustang wheel help

[pro-touringAK]

Hi, I'm new to the pro-touring scene and I have a 68 mustang coupe. I am interested in buying Coy's wheels C-5s however I'm not certain of the wheel size or tire size. I'm wanting 18x8fr and 18x9.5 on the rears with 235/40 fr and 275/35 rr. Currently, I have stock suspension however I will want to lower the car so that the fender lip sits in line with the middle of the tire (from side view) My worry is that the wheels may be to wide therefore going to 7" and 8" with 225/40 and 245/40. Does anyone have any input or comments that could help a rookie. Oh the 8" would have a 4.5bs, the 7" 4.25bs and the 9.5" would have a 5.5bs.

thanks for any help or comments
Brian
sorry so long