'76 Chev sb 400 fouling plugs and crushed headers

Grabbed a 1976 Chev 3/4 ton with a small block 400 in it this fall, and it's supposed to be my daily driver over the summer but is having some problems i've never encountered before. It fouls plugs like a 2-stroke dirtbike. Ran great when we test drove it, but i'm guessing it had brand new plugs in it. I put temp inurance on it for 3 days and drove it as much as I could. It didn't foul up too bad, was still running on *most* of its cylinders by end of the weekend. But if you give it good time to warm up, as in just let it sit and idle.... fouls up fast. Runs on a few cylinders, backfires like crazy, then doesn't run at all, sparkplugs black. Other than that, when the motor is under power and running it's really nice to drive. Lots of power, not tired, idles like a dream at stop lights. I wouldn't say the motor is worn out at all.... just fouls plugs.

What could be causing this issue?? Carb problems? I really don't want to face the possibility that my motor is completely shot.

Also was looking under it... after I bought it of course..... and saw that some of my header tubes are squashed pretty close to flat at the bottom. Truck was used for mudding and 4x4ing, someone obviously ground it over some rocks. Could squashed header tubes maybe be the cause of some missing on those cylinders?

Thanks in advance,
250girl