There's actually a formula for working out carby size
Which is Max rpm x cubic inch divided by 3456 = cfm
So a 308 x7200 /3456 =641.666cfm and @ 355ci
355 x7200/3456 =739.58 cfm
By this you can choose what size you need .a 725 cfm would be my choice .A 355 would rarely see 7200rpm and I think would be great.On a 308 would be a tad large as Taily said it would not get the air speed through the Venturi which triggers the timing of the fuel delivery on a Holley .Rochesters use vacum to meter fuel in more precisely with metering rods fine tuning the fuel in.
Carby's like qwikfuel, demon & the newer holley's have adjustable air bleed restriction to allow finer tuning of the fuel timing.This allows for the old case of over carbing of holley's as a lot of older ones don't have this feature adjustment.
Most of the demon style carbs I think you'll find they use holley parts and design for the convenience of available parts as Holley has been around for donkey's.
I personally prefer them due to being less fiddely and most adjustment are made with out pulling the thing to bits to tune.