Can't modify the back cover. The outlet to Y pipe is not the problem, it's the smaller outlet for the return line. You could cut the return pipe down...
To be honest, I don't know of any replacement pump that comes with a plate on the back; they're usually sold separately. You'd typically clean up and...
If you want one for a bolt on pulley, look at the 1987 Volvo 780 Bertone 2.8L water pumps on rockauto, amazon or ebay. Same thing as the DeLorean for...