Worthwhile upgrades to stock '73 340?

If the shortblock is worn as the headas are, and you put fresh heads on it, it will have issues pretty quickly in terms of oil use and ring seal issues. My advice would be to leave it alone with the exception of a better cam and timing set, and add the right matched valve springs and seals. If there is money in the budget for a rebuild, do it all properly. If not, don't spend just to spend. Clean it, cam it, paint it, add the headers and duals, and save to "do it right".