http://www.turnswitch.com/index.htm Try these guys. I'm not even going to get into the ohm rating thing, it depends on the head unit anyway, but they seem to know their stuff on speakers and aren't totally out of line on price. Your car was setup for a single rear speaker and a rear defroster and that is why the different size. Another 73-4 oddity of WTH change something that works.You could also use readily available (ebay) plastic speaker spacers to space down the 6x9s and drill one for the 5x7 hole if there is enough material.70-80s GM big cars are good sources for low buck baffles, you can even use the center post mount by using epoxy to mount a bolt to the magnet. Gives a factory look to it. Just thought I would share this because its a subject I've been putting some recent thought to for my 74.