A Barry Grant 850 Double Pumper may be a bit large for a 502, but not by much if it's set up properly. I ran a 780DP on a 400 inch GTO, and it ran . . . well, like a GTO. 11.80 on MT 8" wrinklewalls. The engine was pretty radical, though . . . 12:1, Ram Air V cam/valve train, 4:33's, Muncie, etc. I also ran an 850DP on a 454 in my 30 A-bone. 9:1 engine with Comp Cams Magnum 292, top-loader and 4:10s. It was a little over-carbed, but it tuned out fine.

Your BG is essentailly the same as my Speed Demon. The ported vacuum should be the small brass tube, furthest to the front, on the right side of the baseplate.

I've always been a fan of mechanical advance distributors. Both of those cars had mech advance. Get the right curve, wind the engine up to about 3000, set the top timing and let the initial go where it goes. Not much for mileage, but real easy to tune.