I do wonder Phil, whether you'd be better to source a heater from a later ('74 or 5 on) heater. As you'll be aware, the old round heater wasn't known as a fug stirrer for no reason!
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '11 MINI Countryman Cooper D All4 '90 LR 90 SW