I have not found a side pipe with a 4-hole flange that bolts up to the 4-bolt flange on the Classic Roadsters' header.
I did, however, find a flange gasket that looks to be the correct one:
COLLECTOR/CONNECTOR Flange Exhaust Gasket
I figured that when my car's headers finally rust through, I will probably order a set of these headers and intermediate pipes from Vintage wheels, and just re-weld my existing flanges, mufflers, etc to them:
side pipes and headers : Vintage Wheels, Mustang, Hot Rod and Muscle Car