1986,
This is sort of a tough question to answer. At the simplest level, the 700R4 has a .7 Overdrive, compared to the direct (1) 3rd gear in the TH350. That means your engine is only turning over 70% as much at a given road speed. So, you might anticipate a 30% fuel mileage increase.
However, this is dodging the bigger question, that no one really has a firm answer to -- is the fuel useage vs RPM curve of your engine really linear? In other words, if you go from 1000 RPM to 2000 RPMs, does your fuel useage double? Most people agree it doesn't, but I've never seen anyone who was able to give you any formula or data to prove this.
So, to summarize, we don't know how much it will save. 30% seems to be the maximum it COULD save, but more likely to be about 10% to 15%, depending on all sorts of factors like driving habits, traffic patterns, engine components, etc.
I hope I made this clear as mud for you!
Brian