I know you talking about a big block, but I think a stroked small block would come out cheaper and make just as much power and your car won't handle like a nose heavy pig!
With a Street demon like a thermoquad, you can go bigger because of the air valve secondaries only open as much as needed. Chrysler put 800 cfm thermoquads on 318's. I'd go for the 750
I would like to add I don't have a big block, I'm assuming the difference in height between the M1 and the RPM would similar to a sb. I'd do some measuring to be sure.
I also have a Challenger with an R/T hood. Although it has more clearance then your tippical A body, you most likely have to use a drop base air cleaner if you use the RPM with a spacer.