Just wondering does the coolant have to be filled all the way to the top of rad.
Reason I ask when I do this. After a run it a little drips out .cant see where its coming from .if I leave it a little lower it won't leak out.
Has the heater matrix been cleaned out & checked as well & are the operating rods operating correctly? The matrix is constantly hot & the cabin heat controlled by airflow via the various flaps. These along with all the foam seals being deteriorated in the heater casing will drastically affect its performance. A fiddly job to put right but extremely worthwhile.
Also check under the dashboard behind the shin bins (storage lockers) for the aircon integration ports. If no aircon was fitted then they were taped over but the tape falls off over the years & allows great amounts of cold fresh air to flow into the cabin.
It's also easy to get an airlock as the heater is the highest part of the system. I fill up from the upper heater hose until coolant flows out from the pipes.
It's worth checking the small pipe from the carburetter tower to the radiator is clear too. You can remove the hose at the top & run a 1/8" drill bit through it to ensure this.