Doesnt have Chevy lifters does it? With some solid cams, the base circle is too low to use the Chevy style lifters. Check your part numbers and call your supplier. The oiling groove is too far down and it bleeds all the pressure off. If the oil gallery plugs are in the back like they have mentioned on here, I would pull the timing cover and check the front gallery plugs. If they are in , then I would check the lifters. Also, even if the pickup fell off, if you overfill the engine about a quart, it should still build pressure.(if you get pressure doing that, then that is your problem. And it isnt usually a good idea to stretch the relief spring. It can build too much pressure when cold and blow filters off, gallery plugs out, etc.
Good luck and let us know what you find.