Larry - I'm not a racer, so not gonna have much to offer. The only thing I can offer is that the wheel isn't all that important, except as it has to do with what size tires you may have available to you. You may have more options with a 15" wheel. You'll find the exact same OD and Width on both 14 and 15 wheels, though 15" wheels may allow larger tires than you can get as a 14"and vice verse. I think you'll have to be more concerned with how much tire you can get under the wheel-wells combined with wheel offset.

Here's a cool tire/wheel comparison tool:

http://www.rimsntires.com/specspro.jsp

I think Merc Marquis or similar had rear discs. Whether they are the right width, or will need shortening or leaf pads added, I'm not sure.

Sorry couldn't help more.