40 PSI at 4k that sounds low to me, does the pressure go up the higher you rev it past that? My understanding of Mopar oil pumps the pressure/flow jumps up rapidly when the engine is revved (unlike Chebbies that slowly build pressure consistent with RPM); it should be giving you max pressure at lower RPMs and leveling off as the revs climb. My $0.02
EDIT: Yup those lifters look like they'd cause a big time pressure loss, the oiling passages in the lifter bores are meant to be covered up fully by the lifter body, I bet you will see oil gushing out the lifter bores if you prime the oil pump with the intake off.
