The MGB does not have a recirc on its heating system but during the oncoming winter months one may be useful.
A simple and easily reversible mod is to tape over the air vent – the one in front of the windscreen. I used clear tape but you can use any colour of your choice and then trim off any excess.
So the way it works is that instead of air blowing through the dash vents they suck warmer internal air into the vents when you put the blower fan on. The air is then forced through the heater matrix and then back into the cabin via the footwell vents. This means the heater is heating up warmer air from inside the cabin instead of cold air from outside resulting in hotter air returning to the cabin!
In summertime simply take the tape off the vent and ……. back to normal.