QUOTE=Henry Rifle]I"m not an expert on carb sizing. I only know what I've run on various cars for the last 40 years or so. A 750 may be a bit large for a 390 with some decent power,

I tried an early 3310 (780 CFM) for a while on my long gone '79 F350 years ago, thinking bigger is always better. It ran like crap, especially in the Colorado mountains pulling my 30 foot 5th wheel. At sea level, ran OK, but the original Holley was about a 550CFM, if my recollection is good. The 3310 gave me about 11 mpg, the original about 12.5 empty. Got an idea and took the Edelbrock Performer 600 cfm off my street rod and put it on the truck. Performance vastly improved, and better yet, the gas mileage went to 13.5 all around driving. And I could smoke the 12" wide rear tires again. Mind you, this was on a mildly modified (cam, exhaust and dist) 460 CID, 6200+ pound F350 SCab.

My experiences with a 950 3 bbl were interesting on my 390 '68 Torino. It ran like gangbusters for about 6 months, then just plain wore out. Shafts were sloppy, leaked everywhere, power valves blew out about once a month, etc, etc. Finally sold it for $65 - and regretted it ever since as it was a conversation piece worth more than that. I then ran a 406 Tri Power on the Torino for several years afterwards - and finally sold that setup a couple of years ago, long after the Torino was scrapped, probably to make steel for ricers