You should check the clearances of the mains and rods both. This can be a serious situation. Take your time and check each one. It is possible that you could have received the wrong bearings. I was in a hurry once and did not double check eveything before installation. The crank had been turned .020 and they gave me bearings for .010. It came in a sealed kit so I fugured that everything would be correct. WRONG!!
Check each rod and each main. You definitely have a problem somewhere. 40 PSI on a fresh engine is not very much pressure in my oppinion. The fact that you only had 40 PSI at start up indicates that you have excess clearances to begin with. Excess clearances, especially in a connecting rod will let a bearing "spin" in the rod.
I don't know what your engine building experience is. Double-check to make sure that you didn't put any rod bearing caps on backwards that might have caused a bearing to spin. Personally, I prefer to go with a high volumn oil pump. I like a lot of pressure.
Just curious, did you install your cam bearings yourself?
Ed