Hi Colin
Grant is correct if you go the remove the transmission and bell housing in the truck route you will have to support the engine carefully to remove the bolts from the engine mount.
This is not really a big issue just takes planning. One other thought is getting everything back in place and correctly aligned. If the engine and bell housing are original or paired set you won't have to worry to much about alignment of the bell housing to the engine as they will have alignment doles if they are not a paired set then you will want to check the centering of the crank to the bell housing, see the manual on this.
Here are the photos I was talking about, because of the slight angle of the Chevy engine in CMPs often makes it so a leak up front may drip on the floor far back on the engine. There is a lip on the front of the bell housing shield that sticks out and catches anything flowing down the front of the bell housing.
Until I carefully followed the leak path, I thought there were two leaks one front and one rear. In all my years of playing with these engines I have actually only heard of one of these plugs rusting through and that was on a CCKW that started loosing water/coolant ant the rate of gallons to the mile. Friend made it home with the truck by refilling the radiator several times.
If you have confirmed that the leak is positively coming from the rear plug then I'd go with pulling the whole unit.
Cheers Phil