I have installed many a rear main seal, First the engine will have to either be pulled or raised enough for upper oil pan removal , you can do it on your back if you raise it enough for pan removal very messy job and somewhat difficult loosen the main caps and take a punch and tap the seal out from the upper grab it with a pair of needle noise plyers and pull it out, Now I would suggest that you put the seal in the oil pan first leave it flush, now take the remainder of your new seal an push it back into upper half leaving equal lengths on either side of about 1/16 that will crush down when you reinstall pan. the seal comes as one complete peice and you have to cut it, so make sure you don't cut it short, leave access for upper half. take you finger and put a little bit of oil on it to help it seat., now if your going to do all this I would check my main and rod bearings and roll in some new ones torque them and do one at a time then you will have a new lowere end and better oil presure, , the numbers on the bearings will give you the size you need. Good luck, Jerry, Master ASE certified Diesel Mechanic, ps if your doing it on your back wear some safety glasses to keep the oil out of your eyes, lol