What is the best way to get an overdrive auto in a 67 Barracuda?

It could be just his opinion, he is actually a very competent builder and knows the owner of Extreme very well, he build transmissions for a couple of the tv street outlaw guys and a bunch of local racers here Drag and stock car guys, very reputable. I wanted to see if anyone had their own experience so far just for my own research, it's a big commitment work and money wise. Trying to weigh out the pros and cons.