I noticed on your picture of the adaptor/pump assembly after you removed it, that you were using some bolts with what appeared to be very large heads. The right hand bolt head in particular, has one of the points protruding out over the edge of the plate. If this is the case, it would prevent the plate from seating tightly against the block.
On my adaptor plate installs, I use a standard 3/8" bolt which has a 9/16 head and there is no interference with the lip of the block. In fact, when you are ready to install the pump/adaptor combo, the bolt heads are inside the block and sit on the lip of the block and make it easier to start all your bolts that secure the pump to the block. I hope this helps with your install.
Dave
