Larry, I ended up using one of these:
https://www.holden.co.uk/displayprod...&pCode=080.545
It's double-ended, but you can saw one end off with no problem. Hard to see there's a hole there when installed. It's also a larger size than the Ford motor which is smaller than the original Cobra one.
With regard to the heater doors, I modified the long rearward-facing one by welding a steel section in to enlarge it. The two small rectangular side-facing doors are exactly the same as those on another style of Ford heater - can't remember if it's 100E, 105E, Anglia or Prefect, but I built mine up by cannibalising both. When you come to the demist outlets, I believe the original elbows may have been metal (can't remember Dan's photos offhand), but you could use Austin Healey elbows which are rubber and easily available. Same shape and easier to work with.
For the fan, I gave up trying to get the (fairly useless anyway) bakelite one off the shaft of the Ford heater and used the aluminium 4-blade one Holden sell to go with their new motor, cut down to suit. A local radiator shop relocated the heater water pipes for me and tested the matrix. The new fan and motor arrangement actually throws out a decent amount of heat, and once assembled is pretty much undetectable. The really tricky bit is matching the correct shade of grey!
Roger