Tachometer and Cab Hole Questions

[1970GMC350]

Hello all! Forgive me if my questions sound really dumb, I'm a newbie when it comes to all of this. I'm learning as I go along when I'm working on my 1970 GMC and loving every second of it. Now, onto my questions.

I'm wanting to wire a tach up. Can I wire it into the distributor? In the blue circle, it's stamped "Tach Bat". I believe this is an HEI ignition as I couldn't find my ignition coil anywhere.

Forgive the cobbled up wiring in the background. I need to put a new engine harness in. A prior owner didn't do a very good job at splicing and cutting wires.

Also, the firewall holes mentioned in this post
https://www.67-72chevytrucks.com/vbo...d.php?t=722258

Did every truck that rolled off the line have every one of these holes? The hole labeled "clutch rod" in the first picture and "trans linkage" in the second is uncovered on my truck. I want to make sure there's nothing missing on my truck that I'm not aware of and plug that hole.

Thanks in advance for any help!