I did a write up, but I don’t know which thread it’s in. Build thread maybe? Anyway, that little blank is just a piece of plastic. Once you remove the lens, it’ll lift off. Yours being an 84 might be a bit different, but my 78 had a plastic tube and a bulb socket there for a non existent idiot light. I ended up having to dremel that entire tube out, and then cut a slot on the back to allow the wires to pass through, and to ultimately make the clock sit level.
I don’t remember exactly what I attached the clock to, since I don’t remember there being existing mounting points, but I believe I was able to attach it to the same point as the gauge beside it and then it just kind of sat there. It’s snug once the lens is put back in place. Then you just have to drill the lens for the adjustment knob. Don’t do like I did, half ass it first and then try to correct it later, as you’ll end up with a crooked clock and a hole in the lens that’s approximately 6 times larger than needed. I ended up getting another lens that already had the hole in the right spot.
This is also a great time to recondition the cluster. I wiped mine down, then grabbed some orange model paint form hobby lobby and painted the needles. I also went through and cleaned all the bulb holders, bent them slightly so the bulb fit snugly, and then put all new bulbs in. It made a night and day difference, especially being able to see them at night. Anyway, I hope I didn’t make this sound complicated, it really wasn’t. It was just very time consuming. There was a lot of dremeling and fitting to make it turn out right. Well worth it n my opinion though, since I always hated that blank spot.
EDIT:
https://www.gmsquarebody.com/threads/78-k10-custom-deluxe.13651/page-20
There’s the link to my build thread, with pictures of the clock install.