It looks good to me. I pulled the middle plug off with the purple wire for the temp sending unit. It was clean and tight. The fuel wiring runs thru the kick panel and to the trunk.
Your on the correct track on suspecting the instrument voltage regulator as potentially the cause of the low readings
It is over 45 yrs old and who knows how many hours of use
I would buy a new one, NAPA and I think Rock Auto both offer them
I don't remember what the correct voltage is suppose to be going to those two instruments but I think it is higher, closer to 8 volts pulsing maybe
Hopefully someone else with a 1968 FSM will chime in and get you the straight scoop on this
