I've had a couple of those through the years. My favorite was a '78 Trailduster. (Same thing but sold through Plymouth dealers) Mine was a big block. If you wanted to drop a big block in your truck, you'd need to swap the transmission too but I'll bet the frame hasn't been changed much. Just remember to get an engine, or change it, to a rear sump pan. My '78 had a worn out 400 that when I pulled it, I found the oil pan had been modified with a DBH (darn big hammer) to clear the pumpkin on the front axle. Previous parts replacers (they certainly weren't mechanics) had really done a number on it. Besides the oil pan, the plastic fuel tank had a hole melted halfway up the side from the exhaust, the LF wheel was held on by the rotors and the spindle was nearly worn through. The guy sold it with a knocking noise in the engine. Driving it home in the summer, I was roasting. I moved the heat control to cool, and opened all the windows. I mentioned that later when I found the noise and he said, "Oh, I forgot to tell you, the cable is broken and you have to shut the hot water to the heater core off by hand under the hood."
I went right through an 18 inch snow that winter after I got it. I sold it to a couple who wanted to go off roading. They managed to destroy the truck in 6 months.
Honk if you love Jesus.
Text if you want to meet him.