The biggest changes to the '72 and '73 340's was the cast crank, lower compression, and smaller valves.
The cast crank really doesn't affect hp, and it will be just fine up to 400+ hp, so no real worries there.
The smaller valves are still 1.88/1.60, so while you may lose a little bit on the top end, this isn't a huge power robbing change, especially with a stock cam, intake and carb. Start opening up the flow elsewhere though and it can become a bottleneck.
The compression ratio is where you really take a hit. '72-'73 340's had different pistons to get that lower ratio. '68-'71 340's had pistons that came out of the cylinder by .018" (which can actually be detrimental if you want to run aftermarket heads, since most are closed chamber and can present clearance problems.) Needless to say, unless you're going to do a complete rebuild you probably won't be changing the pistons, so you'll have to look at getting more compression another way.
I wouldn't go looking for an earlier 340. You'll spend a bunch of money, and after a rebuild it won't matter anyway, since you'll probably have to replace most of the different stock parts anyway. To up the power, your best bet is to raise the compression. A set of closed chamber heads would do this in a heartbeat (RHS X heads, Edelbrock, or even just stock magnum heads off a later 5.2 or 5.9). To do it without buying any big ticket items, you can pull the heads and have them milled down a little. Depending on your budget, you could even have the larger valves added too. The only thing to remember is if you shave the head, you'll also need new pushrods, and unless you happen to be able to use a standard size you'll have to buy custom pushrods, which will set you back ~$200, in addition to the cost to mill the heads and replace the head gaskets.
Switching in a cam will help some, but keep in mind you still want to match the cam with the compression ratio. Just dropping in a big cam with an 8.something:1 compression ratio will probably not get you the results you want. Run a big enough of a mismatch and you might even reduce the performance.
Also, making some changes of accessories can help get you more hp without getting too deep into the engine. A Edelbrock performer rpm or airgap intake manifold will perform better than a stock 4 barrel intake, so you can get some hp there. Headers can buy you a few more ponies too, even without other changes. Go with a well tuned newer carb and just those changes can move you closer in performance to an earlier stock 340.
In the February 2001 issue Mopar Muscle ran a bunch of dyno runs on a stock 340, keeping track of intake (air gap), carb (eddy 800cfm), header and cam and valve spring changes. They ended up getting up to 400 hp on the dyno after those changes, up from an original 280hp. Now, they were using a '70 340 with a higher compression, so you might not see all the same gains, but you'd better believe that if they can add 120 hp you can still improve things quite a bit without needed to really get into the engine.
Pm me with an email address and I can send you the article if you're interested.
