I stuck a used 300D turbo in my 81 this spring and after getting it up and running I soon found large puddles of fresh engine oil under the car. The diagnosis was not hard. It was the rear seal. I just climbed under there and wiped the fresh oil away and then drove it a few minutes. Went back under it a the oil was just pouring out again. It will get into the front of the trans where the torque converter is, but that is no worse than going on the floor.
I don't know about changing the lower half of the rear seal. I figured the reason the seal failed was because this engine had sat out and the seal dried up. So I just yanked the engine again and replaced the entire seal.
To do that one is a big job. I recall the shop estimating quide rated it somewhere around 30 hrs. I replaced the front and rear seal, trans seal, and all the bolts to the crank. They are stretch bolts and most, but not all, of the recommendations are to just replace them once the torque is broken.
I was able to remove the timing chain tightener and that put enough slack there to lift the rear of the crank up to get the seal out and the new one in. There are pins holding both halves of the seal. top and bottom in place so forget about trying to slide them out. Also the seal comes as a length. You will need to cut it to fit top and bottom. Be sure to cut it the thickness of a putty knife long so that it will squash together both halves when reassembled.
Just my .03 worth. Have fun. This is a big one.