I am upgrading from 14" to 15".
I found some rims with an HM code on it.
I tried a test fit and they seem to rub on my 4p calipers. I will probably try spacers.
I think it is a 15x7 rim. Not sure which trim rings to get. Haven't tried my center caps yet.
Any advice would be great.

Interesting. Are these GM 4 pistons or aftermarket. I have 14X6 JC Rallys on my car with single piston calipers (69 style), I had to grind a little off the brackets, but they clear. Is the clearance issue is on the front of the wheel hence spacers.

They are the original factory calipers. The back of the rim is touching the caliper.

That's what I thought. Too many variations Rally wheels. With the factory calipers you should be able to install factory Rallys on that car. Are you dead set on 15" wheels.

I need to buy new tires. I have the 15" rims. Just have to paint and get trims. I figure it will be better than the 14"s.

Those are in fact 15x7 and were used on all sorts of Pontiacs, including Firebirds from 73-81. If that is worth anything to you...

Either that or the HMs were only used on 73 Firebirds. Hard to tell what the site means. Either way... 15x7 Pontiac Rally2. 70s.