I would run a Edelbrock performer and a 570 Street Avenger carb. I put a 570 on my freinds 67 bronco, hes running a mild cam and port 4V windsor heads and it runs its ass off. Much better than the 600 straight out of the box. The 570 Holley street avengers are good stock to 300 hp too.
Purchase $500, Bought him my uncle with only 88K miles ran perfect. Engine $1500 explorer intake $75 posi rebuild $55 ring & pinion install $110 headers & H-pipe $280 exhaust $300 basic main. tires, brakes, etc. ? rebuilt tranny $free
Id say around 4 or 5 grand. I cant remember everything.
Have the heads pressure checked. Sometime heads crack and you cant see and magnafluxing sucks. Magnafluxing only shows cracks that you can see, its not going to show you cracks in ports or oil galleys.
Im not trying to act like I know it all. I havent built a lot of boosted engines but with my experience when ever I helped my boss build anything with more than 25 pounds we o-ringed the head and used a copper gasket with head studs. We never had head gasket problems. Copper gaskets can be a headache because they require retorquing. If you're using good gaskets and they keep blowing than you either need to mill the head flat or try something else. Good luck, I hope you figure it out.