Aft radial seal BAD

Worst possible failure the 1978 300sd I bought for my son just had a catastrophic failure of the rear main seal! We confirmed this with dye test looking thru access opening on transmission. It went from a few drops a day to a quart every 8 or 9 miles. We have never attempted a repair this big our selves. Motor has compression all right around 350 to 375 200k on the odometer. Not sure it is worth the huge effort to repair if it is the upper half of the seal that requires the crank to be removed. One we never have done such an extensive repair it may be above our ability. Also cost with new bearings and bolts, I have not priced them yet but when you add to that special tool required a torquing angle set? Do you really need one? Can it be rented? I guess what we will do is pull the motor drop the upper pan and hope there is a large chunk of seal missing so we could just replace that half of the seal. All in all very discouraging! The only good news is we bought a hard copy service manual off ebay. If it was only as easy as reading the manual makes it sound.
Dave