Phil looking at pictures on the internet looks like the heater box attaches on the passenger side, so if a heater no passenger vent? Been too long since I had my non A/C 64 to remember. Alos a big hole in the cowl besides where the A/C box attached.
Here's a couple photos of my 63 firewall. I'm quite sure 64 is the same. Then a photo of the vent housing, which is plastic on 62, 63 and I believe 64. The is a passenger vent on non a/c cars. The heater box doesn't reach over to the kick panel on the inside, a crappy picture of my 62 inside. 62 and 63 have cardboard kick panels and metal grills. 64 has plastic kick panels and may have the grills as part of the plastic, I can't remember now.
On a '58 Impala, what kind of grill that covers the smaller hole on the left side? Does anybody have a picture of that grill or have an extra one?
I'll make a delete plate for the heater hole then drill the firewall connector fittings when I install an A/C-Heat unit.
Here are the holes, actually 4 holes if you count the one into the cowl and 3rd one from the inside kick panel . Last one how we zip tied the core support on the pull it and the fenders out of the interior to haul it!