Ray,
I get approached by more than a few would-be customers who want to do the same thing you want- they want to basically do a head/ cam upgrade on their 400 or 403 shortblock and make around 350-400 hp and 450 ft-lbs. This type of work typically costs around $2500-3k, and while it is possible on a shortblock that has good compression I generally recommend against it. Why? You might think your engine is in good condition, but the new parts will reveal any wear on parts like rings or bearings, put added stress on them, and not give the longevity and consistent performance a fresh shortblock would deliver. It's hard to say when your newfound power would fade (depends on the mileage and overall condition of the shortblock), but when it does you will not be happy, and be looking for a refund or the mechanic to make repairs to restore the power. If you can live with this "temporary" power upgrade then a head/ cam upgrade should be enough (for now-lol).
As far as head choice goes, 350-400 hp can be easily attained with ported iron heads. Cost to go through a set of iron heads varies from state to state, but here in Socal it's not cheap- my customers spend around $1200-1500 to get a set of heads ported, port-matched, machined, and fitted with stainless 2.11/1.77 valves. A set of stock aluminum heads only cost $500-600 more, so alot just end up going with the latter. Aluminum heads also flow around 260-270 cfm out-of-the- box, so they end up being a much better value in the long run. Stock KRE D-ports or round-port e-heads will make around 450-500 hp with the right cam, so it all depends where your power goal is at. Again, putting fresh heads on a tired shortblock is almost a guarantee for disappointment down the road, so think about rebuilding the shortblock, as it is the foundation of your engine.
Geno