GM used 5 on 5 bolt patterns ever since the 30's, Pontiac in particular. My 40 Pontiac cam with them and Chevy 1/2 ton pickups and van's used 15" in the late 80's. The big GM's Impala, Buick, Pontiac, Old's etc.
The newer large Impalas had 16" 5 on 5, I've got a set on my GMC Safari van that are alloy.
You shouldn't have any problem finding steel ones. good luck
Chevy trucks went from 6 lug(except 4x4) to 5 on 5 in 71' also disc brakes. Most Gm cars were 5 on 4-3/4. but some of the bigger cars had them here is a list of GM 5 on 5
Thanks guys, the reason I asked is I can not find any 15" tires tall enough, I want 32" tall tires and I can't find any 15" tires tall enough, I used to run 235/85/16 on my work van and they are 32" tall so if anybody knows of a smooth tread 15" tire that tall I'm all ears. I really don't want to run bias tires.