I am a HUGE fan of the 250 six, but in this case John is right. If you want to run an inline six in a 3/4 ton, you'd best go with a 292. That way you can bore it .060 and have a 300 inch engine, plenty of cubes for a big truck. Otherwise, I don't think you could get the raw grunt ( torque ) to keep you happy. You make horsepower with a small displacement engine mostly by spinning the crap out of it, and the need for lots of revs won't work well with a 4500 pound truck!

Good Luck, whatever you decide.
Boo Radley

Paint it black with a rattle can, and drive it like ya stole it!