[GMCNUT]

Here is a few shots of the seats and AC controls etc.

So 1961 was an odd year for Chevrolet cars in that the Cool Pack under-dash unit was not the only AC option - in 61 you could get something that while still technically under-dash in basic design, Chevrolet would call it "in-dash" AC, and this 1961 odd-ball AC design for the Chevy Impala utilized a single, bolt-on Drivers side AC vent outlet made of fiberglass and adorned with a beautiful chrome plated air diffuser to match the evap case mounted passenger side diffuser.

GMC was already separated from Pontiac and re-aligned with Chevrolet by the time 1964 rolled around, so GMC borrowed the 61 Impala drivers side AC vent as a simple low cost method of overcoming the dash design difference between GMC and Chevrolet trucks. GMC did not need to have room to send AC out the dash, so this 61 Impala vent worked perfect. A slight modification to the Chevrolet-designed truck AC Evap case was made to send the air to each of the left and right vents - a 2" hole was sawed into the forward or drivers side facing side of the evap case, and the drivers side AC hose to the vent connects there. The Chevrolet evap case design has a large 4" or greater sized square hole on top to accept the Chevrolet-only upper AC diffuser assembly; this large square hole was simply capped off (similar to capping off a chimney on a house) using simple sheetmetal that was then pop-riveted to the plastic evap case. another 2" hole was sawed in the sheetmetal cap and the passenger diffuser assembly gets AC from there.
I really like the borrowed 61 Impala vent design and think they really look great in a GMC - they are more functional than the Chevy diffuser design too, in that the driver, who is many times the only passenger, has easier access to cold air from the wider Impala diffuser to his left than the center vent on a Chevy which is harder to get AC pointed to where you can actually feel it when on.