Fabian, I used the combination of the stock 3-speed overdrive for many years in a pickup with a 3.90 rear end. I connected the solenoid so I could operate it in every gear unlike the factory. Essentially it made it into a 6 speed. This is not generally known or recommended to use a first over but when sometime driving beside another car I would do it just to mess with their mind. Going thru 6 speeds in the 70's was a gas. However using 2nd over was great for climbing in the mountains and having the 3rd over for crusing was great.
I can't remember how to do it but a 60's GM OD unit could be made to fit any Saginaw 4-Speed unit. This would give you 8 speeds. The Vega had a 3.10 1st gear it would almost unstallable from a light. A buddy put one of these in a column shift car and used the tree for all 4 forward gears and had a small lever by the seat front for reverse...Good luck
