Not a hard job at all if you have access to a lift. Otherwise I consider pulling a driveshaft a hard job laying on your back. But than again I am spoiled with lifts. Here are the steps to do this and they worked very well for me. Some of it are compliments of Rick 91GT.
Taking out the Old Seal:
Drill a small hole in the outside of the old one and screw a screw into it. Make sure not to contact the metal surface in the block or the crank when doing this. After this get a pair of vise grips and pull on the screw, the seal will pop out.
Puttin in the New Seal:
Clean all of the surfaces WELL (inside surface of block and crank) with brake cleaner. Use a small amount of rtv around the outside of the new seal to help keep it in place. Rub a THIN coat of oil on the seal and the crank if possible, making sure to not touch the inner surface of the block since the rtv needs to adhere to this.
Now here is the trick to getting it started!!!!!
Take the seal and line it up on the crank the way it's going to be installed, catch the bottom of the seal on the crank FIRST about 3/4" of the way on, than push the seal up to compress the bottom of it. This will make enough room to let the sides and top slide on all the way now. Once the seal is on the crank and straight, press it on with your fingers the rest of the way if you can (mine slid on this easy because of the oil on the inside and rtv on the outside both acting as lubrication). Push it on enough so it is even with the outside of the block. Now you are done!! I would let it dry about 5-6 hours before driving to let the rtv setup.
I'd recommend investing in a Teflon rear main seal, if/when you get around to doing it. You put them in dry (no RTV or oil), and they come with a nice little plastic ring thingy that helps you slip them on there. Mine hasn't leaked a drop since I put it in.
Hm, I dont know. I dont have access to a lift ...just jack stands really. I have a lot of time and garage space to spare though. What level of difficulty would it be to replace as opposed to the difficulty of doing my own heater core swap? God knows that was hell on my own. Even with a walkthrough.
I changed the trans in my car at 10 pm with only one side of the car jacked up. Or just the back jacked up, or just the front. If you get the car 2 feet up on jack stands, its not a hard job at all. Fox bodies are not hard to work on at all. I would say doing the heater core is way harder. Its just a puzzle when doing the trans. Here's what I do. Coming out, x-pipe, driveshaft, shifter, speedo cable, trans electric connectors, trans x member, then the four trans bolts, then get the trans out of the way. If your doin the rear main, then take off bellhousing, clutch etc. Then reverse the order for re assembly.