I've done this once, to adapt a Carter BBD 1 1/2" from a 383 Mopar in place of a Rochester 2GV on 318 Mopar. I started with a 1" thick slab of plastic. I traced each flange on each side of the plastic. I had to offset the flanges by about 3/4" in order to allow for the studs/bolts to miss one another. I then bored several holes through the middle, and removed the stock with a rotary file to produce an oval opening for the fuel/air mixture. I drilled holes for the mounting bolts and studs. The bolts for the carb were pushed up through their holes, and then the adaptor was placed onto the studs on the intake manifold. The carb was then placed over the bolts and held down with nuts. I used the appropriate gasket on each side of the adaptor. The plastic came from a cabinet and countertop shop. It was easier for me to work with the plastic than it was to work with aluminum, and I already had the plastic.
Hope this helps,
Bill
