I'm having a lot of issues with the column shift linkage getting stuck. Would like to switch to a floor shift for the 3spd but could end up putting a M20 4spd in down the road. Does anyone know if there is a floor shift set up that would work with the 3spd and possibly the 4spd down the road?
Years ago, I used a 4 spd shifter (out of a mid 60'Lemans or GTO I think) on a 3 spd car transmission in a '66 C10. The extra reverse shift lever part was easy to overcome. Just pull it back as far as it'll go & wire it in that position. That way the shifter can't engage it. The 1 st gear position on the 4 spd shifter becomes the revere position on the 3spd. After almost 50 years, exactly how I mounted it is a little fuzzy now. I did have the advantage of being young and broke , so throwing money at the solution wasn't an option. And I worked in my fathers garage/junkyard business so I had parts vehicles available and a welder, torches, grinders , etc available. Now days if I were in your shoes, I'd decide if I had some fabrication skills & equipment to work with. If the answer is yes, then find a used 4 spd Hurst shifter including the linkage & any other hardware that might be included.. Then pull the transmission out & set it up on the work bench & fabricate your new mounting bracket . Make sure it's located such that the linkage lengths work out correctly. I'm sure you could find examples of aftermarket 3 spd mounting brackets on the Web to get some ideas on how to proceed.
Having said all that, I currently have a M20 in a '66 C10 with the bench seat. Engine & trans are in the stock location. It was very difficult to locate the shift lever so that it cleared the bench seat in any position. If you've got bucket seats, like my old C 10 from 1975, then the shifter location is not a problem.
I found my M20 including the Hurst shifter for 600 bucks on CL. Maybe that's a better option for you.
