The "hot air stove" on the exhaust manifold appeared on 1968 and newer GM cars & trucks when they introduced the "CCS" (Controlled Combustion System") to reduce emissions--it consisted of the heated air cleaner ,a 195 degree thermostat,and distributor advance curve modifications..along with a "heat riser" or EFE valve to route exhaust gases under the intake to heat the carb for better fuel vaporization..
Not all 50 states got some of the emission stuff,and over the years much of it was deleted and discarded by previous owners..
Canadian built vehicles often lacked emission controls,I've seen some late 70's trucks built in Canada that had no catalitic converter or any other emission devices other than a PCV valve,and some retained point distributors after USA built ones went with HEI..whether those trucks were sold here,or were snuck across the border,I dont know..most likely they were not "legal" to sell here ?..
I think the cold air ducting came around 1985 or so..
For the record ,I have had several GM cars and trucks that loved to ice up the carbs on a humid day under 50 degrees,to the point they would stall at idle, and sometimes frost up the carb enough to make the throttle butterflies stick open (very dangerous on icy pavement,when it wants to stay at 2000 rpm in drive,and your front tires skid,and the rear wheels keep wanting to spin !)..
..
On more than one truck,I had to hook the vacuum operated heat riser to stay shut during winter months--otherwise I'd have to drive at least 10 miles or 20 minutes to warm it up so it would not have carb icing issues..I had a short 3 mile trip to work,and it was a miserable trip when the carb kept freezing up..
