Motor back firing??

I recently purchased a 51 chevy from an original owner. He had told me its been sitting some 17yrs. It ran great up to a year ago, when he could'nt get it to idle. I brought it home, cleaned the tank and lines, replaced the fuel pump, coil, plugs, wires, cap and rotor, and points and condesor. I pulled the choke on, hit the starter and it fired right up. After about a minute or so it started to break up. I bumped the choke in a bit and that made it worst, I then started to feather the throttle and it started back firing thru the exhaust. And I mean bad, sounded like the forth of july!! Im at my witts end here, does anyone have a suggestion?? <img src='/_siteconfigs/_global/images/community/bbcode/confused.gif'>  If I can't get the 216 to run (which I would really hate) I may have a few other questions on swaping it for a 250. (I'd like to keep the 6cyl old feel)

Thanks for your time  

There's a few different things that could cause your backfiring issue.

When it's cold, and you first start it up, does it run smoothly?  

The first thing I would do would be to put a gauge on it, and watch manifold vacuum.  If you're reading above 15", and not jumping around erraticly, It could just be a case of something as simple as a stuck carb float making it run extremely rich, or some other cause of an overly rich condition.

If you've got wacky vacuum readings jumping up, and down, I'd be looking at the valve train,  Possibly a timing chain that's worn/jumped time, or maybe even a weak/broken exhaust valve spring. or even a burned valve.

When it comes to diagnosing problems like this, the vacuum gauge is your best friend.  Here's a link to a great page that will help you "Read" your vacuum gauge.
http://www.secondchancegarage.com/public/186.cfm

Keep us posted on what you find out.
Ace