[Wbulk]

I glad you asked this question as I am going through the same thing right now. I found there was a lot of old information on the subject from years back, but it must be specific to your exact situation.
These are the issues that I am dealing with:
1. Is the body perfect on the frame.
I found that mine was within an 1/8" in the back but 3/8" off in the front as mounted from the factory. I will have to fix that before I glass things in. This could affect tire clearance from side to side. I decided to level the frame front to back and side to side with shims, and then measure the body from the frame to the outside lip of each fender, and then from a point on the frame up to the lip on each fender.
2. What axle do you have. I have a B and B narrowed 9". You have to compare apples to apples. The narrowed 9" is 56 1/2" total width rotor mounting surface to rotor mounting surface. If you have something else it will effect you tire and wheel choices.
3. Where do you want the tires to set in relation to the wheel lip that's about 1/2" in from the outside. I want mine to set about a 1/2" inside from the lip, but I am not sure if that is enought given the movement in the rear axle in corners. I have the four link with heim joints instead of the rubber mounts. I understand the rubber has more movement in cornering, but am not sure.
4. Was your axle mounting points welded on exactly the same from side to side on the axle and the frame. I found mine was 3/8 closer on one side than the other. I did a lot of measuring on my frame and axle. In a perfect world I would have 6 1/4" from the rotor mounting point to the outside edge of the rear fenders as a starting point.
I really thought I could do a lot of measuring and lay it all out on paper to determine my best options. But because I am waitng for some back ordered parts I can't mount my axle yet.
I believe this is one of the most critical decisions a person makes on his car. If you want the look right the selection needs to be right on; not to mention the costs involved.
For me I decided to either build a gig to simulate the rim and tire or buy one from Summit. I think they are about 59.00 and are totally adjustable.
Remember back spacing is the outside of the rim to the mounting point, so you have to allow for the rim thickness. In theory a 9" rim could actually be 10" total width depending on the brand.
You may also have to cut the inner glass panel behind the seat and reglass it in for clearance; some have had to do that.
Sorry, I don't know much about the fronts yet.
Good luck! Tell me how it turns out.

Wayne