Can't change a 350 into overdrive. Don't think you can even get ANY other gear ratios that'll fit into a 350 (at least not for normal money). Get another 700 trans from a junkyard or second hand somewhere, they're not difficult to find and don't cost a lot. or, as Streets said, rebuild your old one, but that takes some expertise (which I for one haven't got ) And for the 350 you'll probably have to change your driveshaft. Wouldn't go to those troubles, if I were you.
JMO,
Max