Theres a small trick to this; its not as easy as dialing in a manual transmission in my opinion (but then again Im a stick guy).
You can't just multiply 1st gear by the axle ratio and try to get enough gear multiplication to move the car with a rear low enough (numerically) for economy
You need to take the convertor into account, along with the camshaft of the motor. While it may not apply to your specific application, if you get the rpm's too low in high gear, it will make the engine less efficient.
That being said, you also need to look at the type of axle, you may not be able to drop the ring and pinion to the lowest (numerical) set available, as it may take some different parts to install them in your specific rear axle.
I'd say 3.23:1s are a good compromise for the TH350, for a comfortable 1st gear and keeping the highway revs down.
lightweight steel rims, and normal sized tires will help.
A TH200-4r is the same size as most TH350s, and it has overdrive. I would use a 3.23-3.55 in the rear there, again with steelies and normal tires.
Use a quality converter as well.
Also, a huge oilpan is not your friend.