Hi watto1,
Looking at the pic- on the left side base plate front corner, just below the mounting bolt/stud there will be a series of numbers stamped. They could be any of the following-
1403, 1404 = 5oo cfm
1400, 1405, 14054, 1406, 14064 = 600cfm
1407, 1411 = 750cfm
By what I can see it has an electric choke and the engine looks like a 289, so I'd take a stab and say it's a either a 1403, 1400, 1406 or a 1411. For a 289 it should be a 1403- 500cfm. If you use a 1403 with a Performer manifold on a 289 it'll have plenty of power and pretty good fuel economy
Hope this helps!
Cheers-
Layne