Ooops my bad, I assumed you had a 235. But the GMC270s should be somewhat similar in design. Where is the other side of the head line connected to? When needed, it should be connnected to the pressure port that sends oil to the oil pressure gauge and oil filter inlet. Follow your lines. You could also carefully loosen the line at the head, run the pump and see if oil comes out. Thatll tell you if the prob is in the plumbing or in the head.
On engines that had shaft-mounted rockers, the oil flows thru the head, up the oil feedpipe connector to the rocker shafts, thru the inside of the rocker shafts, then comes out a 'weep hole' in the top of each rocker. Then the oil flows down over the valves and pushrods for lubrication. (disclaimer-this is how 235s lube; 270s may be similar)
Proffit claims to be the GMC man, maybe he'll chime in. Step right up, long-motor bunnyman.
:p
