In mid 60s the heater option was the MiniMatic style underbonnet fresh air one I think, it bolts in above the clutch housing. Air came thru the firewall on drivers side and into a plenum that provided air down onto your feet, or up the RH demister slot only.
Look under the dash near pedal box and see it there's a metal flap attached there.
These heaters don't have a fan, but you can do what I did and fit a 3" bilge blower into the ducting, from
www.biasboating.com.au
These heaters also add to the engine's cooling capacity- as they are an auxiliary radiator in circuit all the time.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
