Hi all,
Firstly - apologies for the lengthy post, but given the guidance and advice, I wanted to say thanks and share the results which I'm really happy with.
It has taken me a few weekends but I finally got the heater box and demister tubes done. I was able to clean the light rust with Citric Acid and steel wool (Doug was right - I was amazed), and used Evaporust for the ones that need a bit more effort like the heater fan basket. The parts came up really nice - even the little balancing weights on the fan blades. I can clearly see "Torrington" and "408 Air Rotor" on the bottom. I elected to coat them in a satin finish clear coat to prevent further rusting but keep the 'bare metal' look. Thanks to the forum tips, I was able to reuse the original heater element including the plastic retainers (pic 5) and the original rubber gasket for the heater motor. The small retainer clips on the motor shaft are still there and I'm just waiting on replacement wires from Virginia Classic Mustang to arrive to complete the motor itself as mine had been cut and rejoined.
The heater box itself came up a treat. I was able to fix the cracks (pics 1 and 2). It took a while, but I also rebuilt a part of the broken edging, including the inside edge 'trough' (pics 3 and 4) and the small indent where one of the retainer clips goes using Q-bond and a Dremel. Its expensive stuff, but I cannot fault the results.
I couldn't save the Plenum, however Phil Beresford supplied me with a replacement original, so all good there with just a Scotchguard treatment.
Bob was right about the demister tubes, they are VERY fragile! The tubes themselves are in good condition. There was a couple of pieces broken from the cardboard top (pics 7 and 8 ), however when I (very tentatively - thanks Bob) removed them, the broken parts dropped out of the bottom of the tubes. It appears they fell inside and sat there for who knows how long. I set about putting the cardboard jigsaw back together (pic 9) and the one missing piece was replaced after a '54 year old piece of cardboard' donation from my old (and un-salvageable) plenum which was a great color match (pic 9 top hose). I cut the plastic ends off a Scott Drake set to make a perfect template to ensure it all went back together correctly. A clean up, light dusting with paint for cover (pic 10) and Scotchguard later and they came up pretty well.
For the demister tubes reinstall, I plan to use the Scott Drake plastic ends that I made for the templates and slide them over the tubes to provide support for the cardboard where it hangs from the dashboard - particularly where the retaining clips go in. Essentially the plastic distributes the weight evenly and the original cardboard just rests on top of it with the retainers holding it all in place. You cannot see them and as nothing is stuck together, its easily removed in future. I figure it will reduce the strain on the flimsy cardboard - particularly around the retainer hole area - and hopefully help it last another 50 years. I checked with the retainer clips and it all fits.
Now just need to keep an eye out for the postman so I can finish the motor wiring.
Cheers - and once again thank you for the invaluable guidance and advice.
Stan