67-70 rally dash cluster the heater control mount is cast into the rallye cluster gage housing. Should be no issues there. Looks like the only thing you need to do is fill in the ignition switch hole in the 67 rallye dash, and move your ViN plate to the 73 dash. The 67 only glove box door has a notorious weak spring holding it shut. On hard acceleration they like to fly open. Try to find a locking glove box knob for the 67 glove box door. The top end of the rallye cluster bezel has a flat steel sheet that attaches to the bezel to allow it to be screwed into the dash frame at the top.
Hope this helps
Matt