77' Rear Main Seal Question

[qbrew]

My 77' Long bed (350) has a really bad leak at the rear main seal. Upon my usual mechanic's advice, I took it to a transmission shop. The Transmission guy called me and told me that the Seal wasn't like the ones he normally replaces, and isn't anything he feels comfortable doing. He said it checked with the Mechanic he normally refers people to and he said he would prefer to work with the Transmission connected rather than dropped.

Any idea what would be different, and why the engine mechanic wouldn't want the transmission out of the way? My normal guy is too far away to drive with that bad of a leak, but I may have it towed if it sounds like I may be getting screwed.