After close on 50 years much of the original OEM insulation will have deteriorated and even fallen away in a C3. Critical to cabin temperatures are transmission tunnel and dash/firewall insulation. I haven't added any additional heat/noise insulation other than 'pads' affixed to the underside of the floorpan over the header collectors. My 25 year experience of owning a near to 600 hp BB coupe is that a BB car will always have worse heat dissipation issues than a small block.....bigger engine, so greater heat source, less space around engine to vent-off hot air etc, etc. With today's insulation materials both heat and noise issues can be addressed at the same time. My personal experience is that good quality ceramic coated headers can reduce under-hood temperatures and therefore heat transferred into the passenger compartment. However some brands feature collectors that are very close to cabin floor creating hot flooring.
I have to say that I have engine and cooling system that never overheats - runs a bit 'woolly' in town when hot but never boils over. Cabin temperature is another matter entirely. I avoid traffic jams like the plague. Turn the engine off and re-starts can be challenging. Cabin heat build-up when stationary with engine running makes T roof removal essential during summer months. When finally on the move again the heat then blowing though the cabin from under the dash can be enough make your eyes water!
