Beta or Delta 15a. Those are Eminence's most vintage-sounding speakers suited for bass IMHO, and I think they would rip pretty well in a cab like that. I have Deltas in my B-15 repro cabs, and they sound identical to the 66 CTS I have in my real B-15 cab. Sucks Fender didn't put 8 ohm drivers in them and make them expandable in the first place, but good idea if you can get the cab made.
Ampeg Portaflex Club #1