Don noticed I was missing these bad boys on my firewall. Between sliding every vent control to the right, putting these on, and running the fan, the interior without AC went from almost unbearable to the realm of reasonable.
I just confirmed this on my car as well. Having done the heater bypass, I was disappointed that on a hot day, the cabin was still too hot. My car was missing these water outlet hoses (64214674121). So, as an experiment, I siliconed a rubber firewall grommet from a bin of my Austin Healey parts, into the 1.5 inch drain opening. The closer I looked, the more it was obvious that all the hot exhaust manifold air would travel up this drain hole on the right side and into the heater access. Test was conclusive, dramatically reduced heat in cabin. Next up, I shall replace plugs with correct new water hoses. Ahh, progress.
I also had a new shift boot fabbed a few months ago, and that helped a ton. My heater is INOP, so I might end up doing the heater core bypass anyway.