That Crazy Squirrel
I'm too LAZY to Choose a ... oh **** it...
You could do a lot worse than sticking a 4BT in there. They're good, strong, reliable engines for the kind of things you want the truck to do. And there's a ton of things you can do to 'em to get more out of them. Put enough time, effort and a bit of cash into it and you could be rolling coal and not even really need wheels on the trailer. 
