What Holly carb should I get?

I made the cam comment because I ran that cam in a 302 similar to yours. I ran mildly ported 302 heads, a 4 speed, a holley 600 and performer intake. It was too much cam, although I tamed it a bit by running Rhoads lifters.

It would be a good cam with aftermarket heads, headers, gears and a bottom end that is not afraid of 6500 rpm, but not as good with stock components.

If you are running Ford heads, you may want to look at a dual pattern cam to make up for Fords puny exhaust ports, and look at something in the 268-272 range. The Comp Cams website makes some pretty good recomendations.

If you will notice, as displacement gets bigger, the 280 cam mellows out a bit, so it might be a good choice if you go 347 as well.

the difference between the 4150 and 4160 is the secondary float assembly. the 4160 has a metering plate, not a metering block. the 4160 nixes the idle jets on the sec. side, and the jets cannot be replaced on the sec.
also, most 4160's i have seen are single feed with a feed tube from the primary to the secondary. you're running too much cam for a 4160.
and yes, there are many 4150's running with vac. sec's. get that street avenger 570cfm. highly recommended.