How hard to change oil pump?

Been there, done that - You can do it in the car. Be sure to take heed of the other posts, as they may have included something that I missed. Disconnect the battery at the battery ground terminal, remove the fan & fan shroud. You may also have to disconnect the steering shaft and drop the steering rack. Both motor mounts will need to have the large nuts that secure them to the frame removed. You will need to jack up the rear of the transmission as well to get the required clearance. The trans mount will have to be loosened, and it is a good idea to remove the drive shaft.

I also had to disconnect the cat pipes at the headers to get the engine high enough to remove the oil pan. Jack up the engine with a wood block under the oil pan and watch for things that bind or hoses/electrical wiring that may need to be disconnected. I put a couple of wood blocks between the headers and the frame to support the engine.

Scrape the pan mating surfaces clean as possible - old gasket stuck to the surfaces are a source of leaks. Get a high volume/heavy duty pump, and a replacement HD pump shaft. FMS makes one, and ARP does too. When you install the pump, the funny looking washer thingy goes on the part of the shaft that fits into the hex socket in the distributor shaft. It keeps the shaft in place when you remove the distributor, which you will have to do to prime the pump. Forget to put it on, and the pump shaft can come loose and fall down in the bottom of the oil pan.

There is a one piece oil pan gasket which will help re-assembly if you can find it. If you can't get this gasket, use weather strip adhesive to secure the cork gasket to the pan rails and the rubber strips to the bearing caps. Read the instructions on the adhesive carefully to make sure the gaskets are permanently stuck in place and won't move when you slide the pan in place. Use lots of blue silicone sealer on top of the front and rear rubber seals where they mate with the pan.

Fill with oil, replace the filter. Reconnect the battery, switch the ignition on to enable the gages, but DON'T crank the car. Remove the distributor and use a 1/4" hex socket to turn the pump counterclockwise (same direction as distributor rotation) until you see oil pressure (an external gage is a great help long about now) and keep turning for about 30 sec after you see the pressure come up. A reversible drill is the best tool to use to turn the 1/4" socket. The pressure should come up to about 50-80psi with cold oil. Once you see good pressure, check for obvious leaks, and then and only then, lower everything back into place and bolt down the mounts and anything else you had to take loose. Re-install the distributor and set the timing with the engine running using a timing light (don't forget to disconnect the SPOUT plug and reconnect it when finished) 12-14 degrees BDC is good. Start up and check for leaks, let it warm up and look again for leaks. It took me 2 days plus, but I am old and slow, maybe your granny is faster.