I'm thinking the block is too short... so the rocker ratio is thrown out, putting excess pressure on the lifters, which is loading up the cam & labouring the oil pump.
I'm running a standard 5k block 20 thou overbored (not decked), with shallow dish pistons (not machined), 4k head and the Datsun/chev lifter/rod conversion. The same setup but my block is 5mm taller. With the Tighe 104 cam (.423" lift) the adjustable rocker gear was at the end of its tether.
Where'd you find the extra 5mm for the rocker adjustment?