[Bruce Robles]

I've got a Kirkham with a 484 cu. in. Sideoiler by Southern Automotive. It has Edlebrock Keith Craft CNC stage II heads, Blue Thunder Intake, SA flat tappet cam, 9.8:1 Compression etc, etc, etc. All the right stuff.
The motor came with a 850 double pumper and I switched to a 750 with vacuum secondaries. It now runs like a bat out of hell. Original 427 comp. Cobra's ran 750 cfm with the LeMans bowls which are no longer available.
I think if you are going to run flat out a large % of time, like on an Oval or drags, an 850 would be the way to go. But If you intend to drive on the street with occasional trips to hyper speed, the 750 is just fine.
You'll really only know if you try them both, on your engine.
It's just like racing, you don't what's the fastest unless you try.
We had a GT-350R back in the '60's, and they came with a 715 cfm Holley with the LeMans style float bowls and vacuum secondaries. In a test at Riverside we tried a 750 cfm with mechanical secondaries. The car was faster with the 715. So some times bigger isn't better.

Good Luck