The push rod needs to be pulled out. Most likely it will be bent, so you'll need to replace it. You'll find a lot of them at salvage yards that hang on to older vehicles. If you can't pull the push rod out with a Vise-Grip pliers, it may be bent inside the cover. That will require removal of the timing chain cover. Given the mileage you listed, this would be an appropriate time to replace the timing chain and sprockets.
The push rod is pushed out by riding on an eccentric, or off-center disc on the front of the camshaft sprocket. That is pretty hard to damage, so don't go looking for trouble yet. The return-spring pressure of the fuel pump pushes the push rod back in and keeps it in contact with the eccentric.
There's two, and maybe three ways to hold the push rod up so you can bolt on the pump. I use a hack saw blade to reach in over the pump's lever and hold the rod up until the lever can be slid under it. You may find this easier to do if you use your finger to hold it up while a helper slowly rotates the engine by hand. You'll be able to feel when the push rod retracts as far as possible. That makes the job easier.
I've also seen people wait until the engine is cold, then pack the push rod hole with grease. That will hold the push rod in place long enough to allow you to stick the pump in. The thinking is engine heat will melt the grease which will be carried to the oil filter. This method makes me nervous because if you don't replace the oil and filter right away, that grease will harden in the filter and block it. Oil will have to go through the bypass valve so it won't get filtered before it goes to the bearings. If you're going to try this, consider using Engine Assembly Lube instead of bearing grease. Assembly Lube is designed to coat engine bearings so the aren't dry when starting a newly-rebuilt engine for the first time. That grease will dissolve into the oil and flow freely with it.
Some GM engines had a bolt on the front, passenger side of the timing chain cover specifically for this purpose. Once you have the push rod retracted, run the bolt in to hold it there. That bolt must be backed out again after the pump is bolted tight.
Tuesday, October 22nd, 2019 AT 5:10 PM