Ed's exactly right - it was the 50K FC (also appeared at Gold Coast Nats).
Here's a pic I have of it from a NSW All Holden Day, that shows the tacho mounted.
As Ed mentioned, the wiring that hung out the back of it did not improve its looks, nor (IMHO) the 2" screw in the clamp, clearly visible in the photo above.
I've seen the pods that Ed referred to in a Commodore, too. They appear to suit standard 2" gauges, but don't look too bad if you like that sort of thing. You could also make a pod or neck for it in the bit of blank panel to the right of the dash?
Faecal (my modified FE) used to have the tacho mounted on the ash-tray, but I reversed the rear housing/mount and it's now fastened with the LH instrument carrier bolt, along with the SSR-style gauge holder. It's only a 3" face though, you might find a monster tacho a bit inconvenient in this location
(and yes, ignore the red-line mark. She'll wind out to at least 2,700...)
cheers
RET