Auto to manual tranny swap. 3 speed

[chevybuilder18]

I have seen alot of good deals on 3 speed saginaw transmissions in my area, used but capable. I have never drove a stick and i wish to have one, because limited performance in the automatic drive train, plus my pump in the th350 is going out big time. the thing dies at stop lights, believe this is because the lock up mechanism is stuck on because of a vacuum leak in the line. not only because of these problems am i looking to swap, but because the fear of a stall convertor comes to mind and alot changes. i have heard of 4 speed overdrive saginaws with the same casings as the three speed. if i could (its an optional idea) add an overdrive, it would be much better. but is this possible? again its an optional idea, just wanna get some more info before i go into manuals.