Finally I attacked the heaterbox on a very hot day, how fitting! The box has been laying around for quite a while and I've been reluctant to go at it because it seemed such a complex part. Turns out, it's not rocket science after all.
Again this is not a very sexy part, something I've never seen being worked on in any of the makeover and restoration shows on TV, but as I'm planning on building a car with functional heating and AC it's got to be done!
The box consinsts of two halves, the project is mainly to seperate the two parts, clean everything, replace the worn out foam seals and put everything back together. The new seals come in a handy kit, it includes almost everything you need in one bag, I used high performance double sided adhesive tape to secure the foam.
I was afraid the old plastic of the heater box would be cracky and brittle but its holding up well and should be good for another 40 years of duty.
One half is done, the plan is to finish off the other and put it back in the shelf, as it will be one of the first parts to be put back in the body once it returns from the shop.