340 cam, converter

A mild cam would be fine for the street. There are numerous vendors who produce modern designs to work with today's gasoline. I would have the heads inspected before sinking any money into them. You can stick a lot of money into an old head (surface, guides, seats, springs, valve(s), etc). By the time you get the bill for them, you'll realize that you could have sold the stock X heads to someone building a period correct engine, and used the money towards a good set of aftermarket heads. A rebuilt 904 will handle almost any power level you throw at it if built using the right parts.

A good cam/spring combo and the right convertor will wake that old 340 up. Good luck!