Suggestions on A/C kit w/heat or just A/C add-on

I was planning to add A/C to my 67 Dart project that was originally a non-A/C car. The car is a small block manual trans car. I figured on getting the Classic or Vintage kit that have both heat and A/C. Both vendors also have a universal A/C only add-on that fits under the dash.

That A/C-only kit is less expensive and I suspect the A/C works just as well. Is it an advantage or disadvantage to keep the original heater box? The "full" kits require the passenger vent to be removed. Not a big deal, but perhaps one advantage with the A/C-only is you can keep the vent.

Thoughts/suggestions/user experiences?