Gear Vendors has been around for a long time and make a nice product. Not cheap though. This old overdrive unit in the ad is more of a resto piece or a novelty. It's really not a great option for a more modern vehicle. It's not very strong, parts are expensive, has no first gear synchros, requires several external components to work properly, will need a custom shifter, etc, etc. Here are some other overdrive options:
-T5 5speed. Cheap, compact, self contained.
-MY6/A833 New Process. Very strong, standard sized, uses "normal" shifter, bolts in place of TH350/TH200.
-Corvette 4+3. Push button automatic overdrive, common T10 transmission, "normal" shifter, fairly compact.
-T56. Self explanatory.
-Ford SROD/RUG Toploader. Similar in concept to MY6/A833. Can be used behind a SBC using Jeep (Iron Duke) bellhousing. There is also a BOP bellhousing IIRC. There used to be scattershields that were SBC/Toploader but I don't know if they are still made.