New to mopar.. 340?

So I can get a good running 340. What should I look for to tell if it's really a 1969 340? Also it's going in a 1970 duster and want it to run mid 12 second and still be streeible how hard is that? I was thinking 3.55 gears 727 with 2000 stall and the 340 has a cam and intake and headers but what else do I need to make it 12.5 car? I know everyone says a stock 340 puts out about 325hp 340 torque. So I'm thinking it shouldn't be to hard