Update on my project. So I did not end up taking the dash off. Once I removed all the old gauges, there was plenty of room to reach through the speedo and tach openings. Now I have everything working, but when I turn the key over, the voltage drops to 0 and it won’t crank, click, nothing. Pretty much everything else is working. Any advice is welcome. Here’s a video of the situation.
https://youtu.be/gpRMPlick9g?feature=shared