You think you screwed up. Here is what I did in June of 1970 to my ordered SS 454 / M-22 El Camino. Buy a new tach. Drill and tap the column by the key to mount the tach, The tach is now in a fixed position and will not move. ( I am a tool & Die maker, Yep, I am real smart!) I FISHED the tach wires up through the inside of the column.
Tach is mounted & working the job is finished, LETS go for a RIDE !
My Camino is sitting in my Dads garage , Out of gear. SO,,, I start the LS-5 UP,,,, YEA BABY,,,,,,, Slap that sucker in reverse, ( Yes the back drive is still working) WELL,, OH $H!T..... The steering column starts to SMOKE, the engine starts missing, All I can smell is wires burning,,The engine quit running. I go to un/hook the Batt. I will not live long enough to forget that dumb azz thing I did.