After further thought, I withdraw my blown fuse theory

. I was searching for a single cause of all your malfunctions, but now I think there are 2 separate causes.
Regarding the heater mode selector, the dashed yellow line in the attached image is the vacuum hose for the heater/AC damper position actuators. It passes through the firewall on the passenger side just below the refrigerant lines. This 1/8" hose should eventually connect to the plenum, just after the throttle plate, where shown by the yellow arrow. I suspect this vacuum became disconnected during the intake swap.
As to the reverse lamps, speedometer, and odometer issues, a potential common cause is the transmission harness. The reverse lamp switch and output shaft speed sensor are mounted on the transmission (I do not know where, because I have an automatic). If the connector for this harness is disconnected, it can cause all of the 3 malfunctions you mentioned.