Rally wheel backspacing

So I want to replace my 50 year old 15 x 8  Rally wheels.  Can't seem to pin down the exact backspacing.

Various places list 4.0", 4.25", 4.5"....

Anyone know what it is supposed to be for my 72?

Also, anyone know of a good place to buy new Rally's at a reasonable cost?  I don't want the cheaper 10 hole universal fit Rally's.

Would think the specs would be somewhere in the Assembly Manual.

Bair's doesn't have any wheel specs on their site but MidAmerica does (1969-1982 steel rally wheel-15x8).

Assembly manual is just going to tell you how to bolt them on.

Again, I've seen different backspacing on different parts places.  I had see that at MAM.

I'm thinking 4.25 might be the right answer.

But if you look at their aluminum wheels, they have a 4.5" backspacing.

Confused.

I just measured mine and from the wheel mounting surface to the inside edge of the bead flange is 3 1/2". That would equate to a negative 1/2" offset.

Greetings to those who may be interested.  The December 2021 issue of Corvette Magazine has a fantastic writeup on Roll Models, (wheels), for C1-C3 steel and aluminum wheels. It may answer All of your offset and back spaced questions. 

I don't get that mag anymore.

The article says that the 15x8-in rally wheel had a -11 mm offset in a 3 1/2" backspacing. Basically what I measured last night.  Not sure why they mixed measurement systems but it is what it is.

So that is 3.5 inches from the bead?  That would make it 4.0 inches from the lip....I guess.

It's from the inside vertical surface of the wheel that the bead rests against.  I used to measure from the flange to the mounting surface and that gave me 0 offset.  Need to measure the wheel and see where the 8"  measurement is.  Tomorrow's project.

The 8-in measurement is from the inside of the flange across the wheel to the inside of the flange. So my previous measurements re: offset and backspacing are correct.