This is one I had Installed July 2022 and has worked perfect since. I had the A6. There may be one for the R4 if it has different hose connections at compressor, I'm not sure. I have no regrets since I had several rebuilt A6's since 2015.
https://nostalgicac.com/a6-to-sanden-conversion-kit.html
That is the LO speed for the blower. If you have 4 speed fan switch with no off position. Other years may differ. I do have AC but not sure if that matters.
I have one similar to this pic. (Pic from internet). It came with my truck when I bought it. 74 did not have them. I have an in bed tire mount. If you can use the one I have, you can have it. I don't have any other hardware for it. It has been sitting around for 6 years and probably will if you...
This is how I have done mine. I use to do this to the cars I had in the 70's. Works great for me. I have one on each door.
1st pic, door open, light on.
2nd pic, door open, light off.
A pic of your setup would help to see what you have. In my opinion, I would not think 1600 cfm from 2 fans would be enough. If they were 1600 cfm each, I would think that would be sufficient.
Mi
Mine went out 2 years ago. Fuse holder burnt. Replaced with same type and no more problems. I also deleted the low speed fan by removing the top wire from the resistor connector. Now I have 3 speeds and the low position is off. Works great.