my z28 alum 15" rims (7") are 5 lb lighter than the 15x8" truck rallys in steel..
not enough to write home about..
I'll bet your "feel" of 20 lb was more the tires than the rims..
centerline makes a smoothie alum wheel. but $$$..
your billet wheels also have a big openings. that any alum wheel made like a steel wheel would not have.. remember that..
no idea of the bolt pattern but 70-80 m/b oilburners had alum wheels with hub caps.. the wheel itself looks like a basic steel wheel.
don't think you're gonna see the gains you thing you will with a solid alum wheel