You're car sounds just like mine, original auto i'm converting to a 4-speed.

Keisler is not your only option, look up Passon's 4-speed, looks like a factory 833 and will give you the overdrive you're looking for. Personally, I've heard nasty things about the Keislers so I say stay away but that's me - no personal experience here.

Passon's is nice because there's no modifications needed, with Keisler, there might be but im not 100% on that