I dont think I agree on 308s giving that much more power. When any of the 340/360 head designs gets fully ported, they are within 10% of each other. I dont think they'll give a 60hp boost (at the crank) even with porting just by bolting them on. I've enver seen that kind of result even in pro ported examples. In stock form, the 308 has a faster port, and a better exh port, but in full race ported form, they make no more real power than any other race ported LA head.
AS far as using your #s matchign engine, I wouldnt for 2 reasons. First, in any longer than stock stroke build, you want the thickest cylinder walls you can get. If you are .030 now, you'll need to go up to .040. By itself, not really fatal ina street engine. I've built 2 like that with no issues even with over 500hp. But, the pistons for .040 over are custom, or at least they are catalogued that way, and priced that way. You should not go .060 over IMO for any reason unless each bore is carefully sonic checked. For the cost of another block purchase, and not reusing associated parts, you get investment insurance for the life of the car. A customer I'm building a trans for is doing just that. The car (a two owner '70 340 4sp Cuda he's owned since 1973) is in the body shop now, almost complete with their end of the work. He rebuilt the numbers engine with a nice performance build, and a hotter cam. That is what's going into the car when it's done, for the "show season" next summer. He has a second 340 that will be the crazy 416. There are a lot of things that can go wrong and losing the numbers block because of a problem really hurts the value.
That being said, a 500 hp 418 (340 block at .040 over) is a mild build. That could be supported by race ported factory heads, but that's about the limit. The fact is, they just cant move the kind of air the 4" crank likes. On teh cranks, I've used callies a couple times. They are..for lack of a better term..perfect, in every detail for a reliable high hp build. They are not too expensive ($900) vs the MP, they will take 50% more power easy, balance easier, and are demensionally dead nutz every time. I get mine from Mancini Racing. I have not used Oliver rods. I havent reached the level where I feel the customer will need them. At $1200/set, they arent exactly budget. I use Eagle H beams in the majority of my 4" stroker engines. I wont use SIR, Scat, or others' less expensive rod. I'll use factory rods first. The rotating kit from Mancini runs around $2500 for crank, rods, Diamond forged pistons, file to fit rings, and main and rod bearings. Not a bad deal really. All together, an Edelbrock headed 418 with a modest sized solid roller will make 560+hp N/A on pump gas, and rev to 7000 rpm. An engine like that will cost about $8500 complete, intake to pan, all new except the block casting. (caps would be Pro Grams) You can build a lower budget pump gas with the MP crank for about $2300 less that will make in excess of 500hp under 6000 rpm, with a hydraulic cam on pump gas, or a true "budget build" for about $4000 reusing your heads, MP crank, hyper pistons, and stock rods. Hp would be a guess and my recommended redline with it would be 5800. I doubt your heads are in as good a shape as I'd like them to be valve job and porting wise. If they need to be gone thru, add $1000. These are pretty fair real world estimates. You can cut corners, and ebay shop if you need to. Just be careful..almost nothing you read on there is valid info any more. A solid 340 could be built for less $$, and easily make the 450-475hp level you are asking about too. I'd say depending on what you can or want to reuse, you could spend as little as $3500 or so.
