1975 k10, recent motor dropped in, sat, now wont run

[Hillbillydrums]

Hey there. My truck.was running fine, actually just dropped the motor in a few months ago. It's never been running perfect yet, it back fires, and I havent wired up the elec. Choke yet. So I assumed that was carb stuff.
Anyways, we left to go to Hot August Nights with my 72 c10, and I returned to it not firing up. It cranks, and then it seemed like a cylinder or two would fire, but wouldn't start. And then it always smoked a bit out the exhaust. They are at least 15year old mufflers and the truck was parked for those years. But now it smokes a lot when floored and cranking. Then it'll stop smoking, (still cranking) and then it'll go boom and a ton of.smoke comes out. 
Any ideas? Plugs are brand.new, and all I've done to play with it is try my other old distributor.
Thanks in advance.

[Captkaos]

Floored and Cranking, are you holding the gas down while trying to crank it?  By still cranking you mean holding the key to the start position?

[bake74]

     Your definition of fine and mine are a little different.  Since you installed it, it has always back fired ?
     Have you gone through the basics yet like timing ?
     Do you have any catalytic converters on the truck ?

[thefarmboy21]

Are you sure your plug wires aren't crossed, distributor a cylinder off, or have a sloppy timing chain? Is it getting good fuel? Also GOOD fuel, not dirty/watery 87 octane or stale gas?

[Hillbillydrums]

Floored and Cranking, are you holding the gas down while trying to crank it?  By still cranking you mean holding the key to the start position?

   Floored- yes, held down. cranking as in holding the start.

[Hillbillydrums]

     Your definition of fine and mine are a little different.  Since you installed it, it has always back fired ?
     Have you gone through the basics yet like timing ?
     Do you have any catalytic converters on the truck ?

I pulled it from another truck. Ran very good in it. All I did was add a edelbrock 600cfm carb from my k5, (carbs good, k5 needs a clutch so its non op)  And I think it has back fired a bit since i got it in. More or less when i play with the carb adjusments. Yes i timed it when it ran. I havent again since i just swapped distributors, but i did that after the problem.      And no cats, Just summit racing headers i had welded up to the existing exhaust. with whatever cheesy mufflers are on there.

[Hillbillydrums]

Are you sure your plug wires aren't crossed, distributor a cylinder off, or have a sloppy timing chain? Is it getting good fuel? Also GOOD fuel, not dirty/watery 87 octane or stale gas?

Plug wires i re did when i swapped distributors. And the distributor couldnt have been off before i touched it anyways, it ran fine, then sat. Didnt change.    And the fuel looks good, not yellow or smelly. It was low (1gal or so), so i added another 5 gals of new 87 to it.

[Captkaos]

Verify your on TDC, verify that the distributor is pointed to #1 plug,
Check for Fuel and Spark.

[Hillbillydrums]

Ended up being ancient mufflers just plugged with like squirrel nesting. Who'd of guesed?