External Overdrive???

[impalaboy9]

Ok guys I'm nearing the end of my build I will try to update and post some pics later. When I started I was going to be in town with a few very short trips. Now I may have to make an 850 mile trip on a fairly regular basis. I used the m20 I had laying around and I spent quite a while making the linkage work. I also have a new tremec 5 speed collecting dust. Problem is that the transmission is way nicer than my truck and I'd like to use it on a different project. I have heard about overdrives for the 4speed but I don't know anything about them and the search hasn't yielded much. Can anyone tell me the pros and cons and how they work. Also how do you shift into the overdrive? Currently I'm running a stock 305 but this summer I'm building a 283 with tunnel ram for it. Nothing with ridiculous power the tunnel ram is more for the novelty of it. Thanks guys.