Hi This spring instaled a rebuidt 351M rear main leaks into the bell housing of the auto tranny. Frist checked the intake valley, checked the oil sending unit, valve covers. I then changed the oil pan gasket made sure the corners didn't leak. Now I have the engine pulled. when they assembled it in Mexico they got some sealer in under the rear main cap. I'm hopeing that this was the problem. To answer your question, no it is not hard to change a rear main seal. Remove the oil pan [i didn't need to jack the engine with a 6 quart oil pan] remove the rear main. Remove the seal [ might need to use a punch to start the seal out [be very careful removing the seal, sealing surface here] clean sealing surfaces a little carb cleaner sprayed into the block. Put the new seal in oil and GENTLY push the seal around the crank. If the seal starts to cut on the block go slower. Leave about 3/4 to1inch out so that it goes into the rear cap. That way you do not have the two seams lined up. [less chance for a leak] Clean the rear main cap and retoque the bolts. Reinstal the oil pan. Are you sure it is not the frount seal? Oil drain plug? Dipstick tube? All of these leaked on the old engine. GOOD LUCK