As Steve posted, you should put a copper/brass washer on both sides of the block. They do not have to be grooved.

http://www.classicparts.com/1941-66-Brake-Hose-Copper-Washer/productinfo/71-301/#.VJA7WidlVDE

NAPA and other parts-places sell soft copper/brass washers with different inner/outer diameters and thicknesses. They make the seal on both sides of the brass block, and they allow the bolt to seat properly.