What type of front motor mounts does the engine have? If it's got the two brackets for the "bird leg" mounts beside the timing cover, you need to be aware that the top bolt hole on the fuel pump side intersects the hole for the pump pushrod. A too-long bolt in that hole will cause the tight pushrod situation you encountered, as well as damaging the pushrod. That hole was placed there intentionally, BTW, as you can install a long bolt during a fuel pump change to prevent the pushrod from slipping out and getting jammed against the adapter plate. Just remember to replace it with a short bolt afterwards. Later model engines with side mounts had a very short bolt installed in that hole to prevent a seep oil leak.
While you've got the oil pan down replacing the oil pump, you should probably fix the real oil pressure problem- - - -replace the worn connecting rod bearings. Putting a high-volume oil pump in is like giving a brain tumor patient an aspirin for his headache- - - -treating the symptom, not the cause of the problem. You can't change the cam bearings under a shade tree, which also contribute to oil pressure loss, but rod and possibly main bearings can be swapped without a lot of effort.
Jerry
