Help! SBP to BBP and need help finding BBP Drums

whats the significance?

Plenty. If the regular drum is flat and the Mopar drum is off set 3/4 of an inch the new drum will only be on 1 inch of the shoes and 3/4 will be sticking out behind the drum. A brake drum is designed with about 1/4 inch of play and any more than that the shoes are likely to shear the hardware and go everywhere.