steel wheels 5 x 15 and 5.5 x 15

Depends on the wheel and what disc brakes you have.
Can't give a blanket pass or fail without knowing specifics.

I use stock steel wheels on most old cars.

An example of yes/no is the SBP rallies. The factory 14x5.5 rallies fit over the factory SBP Kelsey Hayes disc brakes. The aftermarket 14x6 SBP rallies will not fit!