The reason I said work with your guy is because he's familiar with performance...lol. It gets difficult when as a builder, one is "up against" a thousand other well-meaning opinion-givers... So I stay with generalizations.
Your builder should have the knowledge. If he says he "doesn't know Mopars" he's either busy and doesn't have the time to research actual part numbers, or he's not familiar with how to make any engine perform. I'm hoping yours is not the latter. A good question is does he have a Mopar small block honing plate? If not and he's not willing to buy one - move on. I recently took all of about 5 hours to research what needs to be done to do a written estimate for a 425hp AMC. I've never built one. But I know what it takes to make those numbers, so exactly the AMC approach of how to do it and part numbers took the time. If you want the business you do the homework.
As for your build - based on my rates and shop costs...
1. Open chamber heads - stay at or below 9.5:1 static compression. Closed chamber iron, assuming the builder can machine to tight tolerances and builds for a squish distance of less than .035 - you can go up to 10.25:1 static. Aluminum closed chamber again with quench distance of less than .035" max is 10.5:1.
2. Heads - any head that flows 230-240cfm at .450 will make the power you want with the right camshaft. Factory iron will need a lot of work and parts. You will spend upwards of $1400 for them, without porting. If you buy aftermarket, skip the iron closed chamber Indy RHS. They weigh a TON and in the IMM package require expensive Magnum-based rocker gear. If budget and iron are your parameters go with the EQ Magnums. At least them you can run stock Magnum stuff, or SBC stuff and they will flow what you need. Best bet is to buy them assembled if your guy doesn't know them well. Last is aluminum. I use a lot of the LA RPMs. They are cheap, work with everything, and are lighter. There's room for you to grow too. But - they should be checked out and corrected by your shop. You'll also probably have to replace the valve springs depending on the camshaft.
3. Camshaft - Work with your builder. If he can't spec one find another builder. You will want something in the 230-240° @ .050 duration, and at least .500 lift with the rockers you're using. I have cams I run. It's not worth debate or argument but you'll need a cam that big at least to get the power from the heads you choose.
4. Assume you will have to buy rockers, and buy shafts if it's an LA based head. Assume the pushrods will be the last items you buy, and they should be measured for and the best ones you can get used. I like Comp Hi-Tech 5/16", or Manton or Smith Bros. 5/16" or 3/8". I've used them all. Get tips to match the rockers and lifter choice.
5. Having a Magnum RPM is good but shouldn't mandate the head choice. It's a wash - build a Magnum head engine and spend $700+ on performance rockers, or buy a $300 intake and $400 rockers for an LA. I like the RPM Air Gaps but the old M1 single plane would work too. Don't get lost in the "single plane will lose torque". This engine will not be at a loss for torque. Headers - you can run manifolds, shorties, or real headers. The camshaft will vary for manifolds and shorties will give up some power all across the board. without careful cam choice. If it were me I'd get long tubes. But that's me.
Edit: - The piston choice will depend on the head choice. Flat tops are probably out of the range because they will be too high a compression for your needs. There are a few different manufacturers - find a dished version that works with the chambers you're using to get what you want. You will find it cheaper to have the heads cut a little to reduce the chamber volume if a custom piston with a smaller dish is required to get where you want it. Pay now and get what you want.
