I'd not heard of a 343 Saginaw but here is a bit of what I researched that popped up and I'm not sure if it's your 343 so proceed with caution.
I hope it helps you..
Here is some info I found and links on the reverse switch concern.
Reverse light switch | Chevy Nova Forum (stevesnovasite.com)
Saginaw 3 and 4 Speed Diagram - Chevrolet Transmission Repair Parts (allstategear.com)
https://www.stevesnovasite.com/threa...switch.659279/
Explode view but grainy
https://www.allstategear.com/Saginaw...ram-s/3238.htm
Back up light switch information
Saginaw 3spd to muncie 4spd speedo gear and shifter help. | Team Camaro Tech (camaros.net)
https://www.camaros.net/threads/sagi...r-help.176726/
Reverse light switch | Chevy Nova Forum (stevesnovasite.com)
https://www.stevesnovasite.com/threa...switch.659279/
YouTube 5 part rebuild series
https://www.youtube.com/watch?v=8f8Oq8Ok6bw