Denny,
on the gmc site they have a photo of an original tool to do that. It is a U shaped extention that goes around the rockers. What I do is undo one of the rocklers, pull up one rocker stud, slide the rocker over and use a socket/universal combo.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!