I’m in the planning stages of new custom wheels and tires on my bone stock 1978 F150. I’m looking at a set of 8 hole polished aluminum wheels. 15x8 or larger. The backspace on my original wheels is 3.5 on 15x6 OEM steel wheels. I’m asking for the correct backspace that would not have a clearance issue. I’m believe it’s going to be a 3.75 with a -18 offset on the 15x8’s. Is this correct? I have to order the wheels and I do not want to guess wrong and wind up having to send them back. Anybody out there with this same setup that can educate me will be awesome. Thanks in advance for your help.