Water pump backing plate

I'm trying to buy a new water pump. The pump that I just removed did not require a backing plate. I bought a Milodon pump at Jegs today that came with a backing plate. (#16230) I told the salesman that I was pretty sure my pump didn't have a plate. He checked with one of the Chevy experts in the back room, and told me that it could be installed without the plate. This doesn't look like its going to work.
It looks like the pumps that don't have plates are listed as "289 HiPo". Is that the water pump I need? When I rebuilt my engine, I walked into NAPA and asked for a water pump for a 1966 289. The one they handed me didn't have a backing plate, and it worked fine. Did Ford use two different timing chain covers in '66?

D.Hearne,
This is what my cover looks like:


This is the pump I removed:


The impeller on the pump I bought is pretty much flush with the back of the case.
I think I just need to find a pump that doesn't use a backing plate and has the passenger side inlet. The problem I'm having is Summit and Jeg's websites really don't specify if the pumps use a plate or not.

You're right about the Stewart pumps. I have several friends that have had great experiences with them. One of my problems is I wanted it today. Plus, I can walk into Jegs and pay cash. My wife sees my credit card spending if I order from Stewart. I don't feel like listening to that.