Been there had it happen to me on my 305 build. You have excesive main bearing clearance. rmove the timing cover and tining chain. Grab the crank snout and lift it up and down an pay close attention to the main bearing and you will see it lift up slightly. My crank was supose to be .010 on the main it ended up being .016 and even with a big block pump i had only 40 psi oil pressure and 5 psi idle.Need to pull engine and mic the crank or even stick in some red plasti gauge and see what it come to. I bet the green doesnt even make contact. Sorry man i now check all my main clearances. I use to just check my rods.