What rpm motor for my coleman powermate black-max 80 gallon air compressor?

I know it had a 6.5 hp motor but I am changing it to a 5 hp and adjusting the pulley accordingly. The problem is I don't have the old moor and I dn't know if I should puta 3450 rpm or 1725 rpm motor on it.w What RPM is the 6.5 hp motor on these compressors?