So, your car didn't have a tach-dash anyway. It was only standard on 428CJ 4 speed cars in 1969 (I believe only R-Codes had them std, but experts insist 4-speed Q-Codes had them too (apart from mine)) but auto's only had them as an option.
The only other cars to have them as standard fitment were Shelby's and Boss 429 cars, and the Shelby's (and maybe the B9's?) had a 140 mph speedo
However, the real trick is that to fit a tach dash, you need to change out the whole dash harness and the engine bay harness and alt harness (unless you do a wiring mod thats listed on the Boss302 forum).
Final caveat - SOME cars that weren't CJ 4-speed cars or Shelbys or Boss 429's got tachos fitted anyway without it being called out on the option list - God only knows why...