I have a 79 k10 with the same setup, Th350 to np203. It was leaking right where the trans met the adaptor for the t-case.
There is a o-ring there that I had to replace.*On the other side where the adaptor goes to the t-case you can use RTV or a gasket.
Here is a few pics in this thread, might be helpful.
http://67-72chevytrucks.com/vboard/s...d.php?t=607897