When Black Dragon was liquidating their parts a few years back, one of the (many) things I bought were replacement rubber pedal pads for my '78. The clutch and brake pedal pads seem to fit fine, but the gas pedal pad keeps coming off. The rubber is thinner than the original one and allows it to slip off from where it wraps around the back.

I tried some 3M spray adhesive with poor results. Would weatherstripping adhesive be the best way to get it to stay?

Clean and sand the back of the rubber to ensure whatever glue you use has a chance to stick. Weatherstrip adhesive should be ok. Good old contact cement (not the water based stuff though) would also work.