here's my .02.. On the original question...if you have the cores, that is heads, rods, crank, and block, then a new assembled performance engine making 450hp is about 8.50/horsepower, or $3800. Thats all else new, internally balanced, complete longblock with pan, timing cover, cam, etc. Add your intake/carb/headers/ignition. in "crate" form, that's around $4500.
On the heads..Doing them yourself can be a rewarding experiance, save a ton, and a good way to get thru the winter up north. It's also bad news if you dont have the tools, or the experience with the heads you are using. It's much easier to hurt performance of the port thru 60% of it's performance window, than it is to make it work 40% better. What I would do is this. Gasket matching can be done by you, and not risk htting seats, or hurting flow in any way. So do that. then, have a good shop do the valve job (5 angle using cutters, not stones) and invest in some bowl cleanup. The first $250 of port work will get you the most bang for the buck. Once you get past that, it's all "X$/hr, how much do you want to spend?" As comparison, a typical set of J heads, totally redone (guides, a few seats, 2.05/1.65 stainless back cut valves, comp springs, retainers and locks, ported to what I would compare to a typical "stage 2 to 2.5" job cost $1100, with the guy supplying the cores. Those make just under 500 hp on a 340, with a big hydraulic cam and 11:1.
As far as castings, X, J, 915s, or 308s would be my top choices. With 308s being #1 in terms of initial cost and port designs in stock form. All factory 340/360 castins can be ported to make over 500hp, using the right package. And edelbrocks cost $1200 to get them in the box. Then it's $150 to correct the valve job, usually new springs depending on cam choice..$125 or so. You do get a better port, and a better chamber, plus less weight, and more potential, but intial cost is higher than a good set of iron, maybe much more if the iron ones have good guides/seats already.
