fattest cam in a 340?

Besides the motor details, is it an auto or stick? If auto, what converter? If you put a big lopey cam in and dont have a 3500-4000 converter its going to be a dog. Decent gears in rear will help a bunch as well. I would set your engine to top dead center on #1(buy a piston stop to do this properly), put some proper timing tape on your balancer. Also while there check your at #1 on dizzy and that the plugwires are in correct order. See if you can time it after this. Whats your plan for new bumpstick, doing it in car or pull motor?
As far as lift goes, pistons with valve relief can go pretty big, bigger then what I'm sure you'd ever consider running for a cam. I have .643 lift on my stroker with valve relieved pistons, and could have gone with more lift. Stock 1973 340 have valve reliefs? I couldnt say for sure on that one.
Good luck with figuring it all out.