I have a 59 pontiac 389 bored and stroked to 447 inches with tripower in my 1959 gmc. It's fun to drive and very capable of working. Best of all the motor was a direct bolt in replacement for the 336 that was in there.
My first 59 gmc had a 270 inline 6, and harbors the fondest memories. I prefer the gmc 6 cylinder motors to any chevy v8. That's why my 1940 GMC 3/4 ton long bed will have a GMC 302 straight 6.
Here is an article written by Jim Carter about my 302.
http://www.oldchevytrucks.com/Showcase/the_king.php