Big bolt pattern brakes

Ok mopar maniacs, I have a 1968 barracuda formula S 383 that someone has changed the bolt pattern to 4 1/2 when I got the car there were no drums on rear. My problem is I have a pair of 10× 2 1/2 drums but they will not sit flush on axle flange because they hit the backing plates about a 1/2 in. Before sitting flush on the axle. I dont know of any 10×2 drums. Has anyone ever come across this? Thanks.