I've run a 671 and a B&M 250 on my crate SBC with a plain old Holley 750. The 250 was overdriven 33% and is a great match for a SBC; unfortunately Holley/Weiand dropped the 250 after they bought B&M. They have a 177 for the BBC, but it is a puny looking thing.
Yea PotvinGuy....thats their idea of being effecient....they're taking a beating for that one because thats one of the favorites....anything smalleer than a 250 looks funky. They make the 71 series all the way down to a 3-71 and smaller with another series....
Those things were used in all kinds of diff. apps., like blowing peanuts and insulation in mfg'ing processes to venting really big plating tanks, etc.
Theres one of the old 51 or 53 series that is wide as it is tall and long. Was really popular on bikes way back in the day....the thing is not much bigger than a bucket, kinda cute in a strange wierd kinda way after seeing the 4-71's & 6-71's.
Anyway, yes, you can run a stock cam, and run about 20% under, and the fuel mileage isn't bad. No, not as good as efi, but not bad. Heres one thing folks don't understand....You don't need to run 2 750 carbs on a blower, and you don't need a 6-71.
Get a 600 cfm 4bbl and a 4-71 on a stock early model chevy hi-perf. 350 with steel crank/rods and forged pistons and give me 4 hours to tune and I'll smoke most of the twin carbed 6-71's out there on the street, and get better fuel mileage doing it.
The Paxtons are better yet, more efficient, cooler running, but don't have that awesome look going for them. And will perform every bit as good as a 4-71.
The only advantage to running the big blowers, is you can run lower blower speeds and not heat up the motor while putting out boost. My old Poncho 455 SuperDuty had a 8-71 on it, you could barely turn the dist. it was so close....and I had it where she'd eat 5 lbs of boost @ 5000, ran on a single Thermoquad, with 3.43's, she got about 15 mpg. That was with a fully bodied 73 Firebird. Now mind you, you had to idle around town and look cool doing that....once you hung your foot off into her, she guzzled fuel....
But I drove it daily for 3 years, rain, shine sleet or snow....if it wasn't Too bad....put a trashbag over the blower to keep the rain and the snow off her. Pump the gas 3 times, hit the key she'd lite off, I don't care if it were 20 degrees outside.
But I did have problems popping the lower gaskets all the time. I'd have to replace them about 3 times a year....that got old fast....