1973 340, want more power.

You have some options for making the car quicker before you start upgrading the engine for power. First off I bet if it's a factory 340 car it has a 727 automatic which will never break but sucks a lot of power compared to a 904. I'd recommend a higher-stall torque converter but since you need to pull the trans (or engine) to put that in I'd look at a 904 or 999 trans. Then gears, my bet is you have 2.76, 2.94 or 3.23s, 340s like to rev more so they definitely need some lower gears in the rear end. You said a tune-up has been done, do you happen to know the timing specs (initial and total @ what RPM)?

Beyond that I think the only upgrade worth your time would be an aftermarket intake manifold and bigger carb which might give you 20-30 HP. If you do a compression test and the engine is healthy you might be able to put a better 'modern' higher-lift cam and valve springs in it without losing too much low-end but at that point you will need to have the converter and gears to match or it will be a turd. Beyond that the engine should just be pulled and rebuilt IMO, or swapped with something else like a 360/5.9L Magnum as 340s have a lot of restoration value but for making power they have no advantage over a 360.