randee actually sells the spacers for that reason i think they are 3/4"
and as for your rear irs set up you can use the big beetle brakes and then all you have to do is machine the outermost ring on the drum and 5/8" off the snout of a early bus drum and then she bolts right up. I ran my old 55 like that and the brakes seemed to work great!
good luck
johnny