Ok, here goes. By now I think everyone knows that I went through a pretty long process picking the heads for my 340 sooooo....
1. Stock 915's (J heads) will flow around 200 cfm at .500" of lift. That's not bad at all. Even with 1.88/1.60 valves, if they've had some port work, they should better that number.
2. Don't mess with the 302's. The ports are too small, and even re-worked they won't flow well enough to beat even a mildly worked on set of 915's. Shady Dell (probably the best porting guys ever) have their 302's maxed out at 246 cfm, even with a stage 3 port. Stock they're only 172.
3. 308's are good heads. I actually have a set on my 340, ported to flow 264 cfm. But, they're still open chamber (mine are 65 cc's), and at 264 cfm they're fairly close to maxed out. But, if you already have a set of 915's that have been ported, the 308's won't offer a huge advantage (vs say putting big valves in your 915's). The exhaust ports are better, but unless you're looking for every last drop of horsepower the ported 915's can flow very close to the same, if the port work is comparable.
4. Magnums- These are closed chamber heads, and come stock with 1.92/1.62 valves. I think that as a cheap bolt on for a moderate to hot 318, they're awesome. They flow better than 318 heads, keep/raise compression, good stuff. But they don't really flow any better #'s than a 2.02/1.60 915 head, without porting. Other than a little compression, I don't think you'll see any big gains. Your ported 915's already flow more, and the Magnums also don't have as much room for porting as I understand it.
5. Save up for something else- If you really want to do better than your ported 915's, I'd look at a set of RHS X-heads. With a set of 2.02/1.60 valves, these will flow over 260 cfm, even without porting, so they'll flow better than your 915's pretty much out of the box (bigger valves). I'll look for their flow numbers, I had them before. I do know they outflow my ported 308's with no changes other than the big valves. They also have a 62cc chamber (closed design), so that will up your compression. If the RHS heads had been out when I was buying heads, I'd probably have them vs buying a set of 308's and having all the porting done. Supposedly they won't take a ton of porting, but for up to 500 hp, they don't really need any either.
http://www.racingheadservice.com/Cylinder%20Heads/Small%20Block%20Chrysler/Pro%20Action/18%C2%B0%20360%20X/Cast%20Iron.aspxOtherwise, you'd probably do just fine to put some 2.02/1.60 valves in your 915's, and if you really need compression maybe have them shaved down a little.
I've assumed that you still have the "low compression" '73 340 pistons and not the earlier high compression pistons that are over the deck. If you're rebuilding, keep in mind that if you do go to the higher compression pistons, you'll want to make sure you have enough valve to piston clearance if you go with a closed chamber head. My pistons are .018" over the deck, so I went with an open chamber design to avoid issues. Not saying it can't be done, just saying I didn't want to mess with it.
