I just did the two rectifiers on a Xenon. I used KBPC3504W rectifiers which have the same dimensions as the old rectifiers. I reused the spacers and mounted them the same as the old. I'm not sure what you'd do if you're missing a spacer. The two rectifiers should be exactly coplanar with the transformer chassis.
I don't know why you'd need a heat sink unless your new rectifiers are intended to go on the component side of the board.