What should I do with my 340 that has been sitting for years.

Marvel's stuff,flat works...(diesel fuel works wonders,don't do it if married,or in a long term relationship...) If it were me: I would do the basics: Some mild bowl porting, valve job,fresh GOOD valve job(not the 150 dollar,done in a day junk...Look for someone who insists on good valve guides,not knurling..) The rest ,is on you..(tear down ,budget,what you want it to be...) Best of luck,a wonderful momento...