Push Rod length for a 351M conversion to 400
Are you running a solid lifter cam or a hydraulic?? If its a hyd. cam, then there should not be any lash at all, the push rod should always be in contact with the lifter and the rocker arm should also always be in contact with the valve tip.
Oil can escape from between the lifter and pushrod if there is lash or a gap between them. But if you're not getting oil out of the rear oil port while turning it with a drill, then the problem is elsewhere.