Ok guys now I'm really confused. I've been thinking of upgrading to a 4-barrel on my 79- 305. Engine is stock except for .030 overbore and a slightly beefed cam from Summit to increase low/mid range torque and gas milage, no rumbling or loping. I have an aluminum spread bore , dual plan manifold for a small block that I got from the wrecking yard, marked 27cc. Now how do I select the correct size cfm carb for the engine. I think that 550 to 600 should do the job, but with smog tests every 2 years and gas prices on the upswing I want to get it right the first time. :?

I'd go with an emissions approved Edlebrock(Carter). Easy to tune even for the not so mechanicaly inclined individuals.

If the manifold is a spreadbore then you need a Quadra-Jet, Edelbrock Q-series or one of the special Holley spreadbores (Model 4165). 600-650 cfm is plenty big enough.

Their are alot of options. Just don't go over 650 cfm. Do a search, their is alot of info here on carbs.

a 600cfm should do just fine. like others have said nothing over 650cfm.

Thanks for the info guys. Is there a preference between vaccum and mechanical secondarys ?

Only use mechanical secondaries if you have a race car only driven on the track, a very high performance manual transmission street car or in very rare cases an extreme performance street car with an automatic transmission. What I mean by an extreme performance street car is a vehicle with a race engine that really doesn't belong on the street. So 99.9% of all street driven vehicles with carbs need vacumn secondaries.