RED58,
You were correct the holes for the carb linkage are already there. Mine were filled in, there are no welded nuts. The holes don't match up with the after market linkage bracket, they are to far apart. I used the hole closest to the accelerator rod hole and drilled a new second hole.
Roy - not sure what year you refer to that you had to drill for the linkage mount bolts, but the '58 under discussion will have the 2 holes complete with weld nuts behind them, in the right location -
on the firewall Caddy, as you can
see in this pic, not in the floor
Bill