Crikey Mark, a couple of days away from my devices and you've almost built the car!
I wouldn't accuse you of being OCD with that diff, but I will try the roller next time I pull out the KBS.
I have a collection of MK1 & Mk2 boxes and have re-soldered a few cores over the years (I pressure test them to about 15 psi which is about their working pressure?). Yes and some high density foam with adhesive on one side (or use contact adhesive) and the boxes come up a treat.
Having said that, I bought a new core for the RS heater box two years back from the UK, thought I would give it a try as they came on good recommendation. No complaints to date.
I could not fit the heater box back in the Capri so I did the same as you Marty with a little caravan type heater. Draws a ****load of amps but I only fitted it for engineering (demisting purposes). I never use it.
Mark, If your heater box is functioning correctly, do you need the bypass tap?