More progress tonight. Decided I may as well do it once and do it right so got straight back into it after work tonight. Pulled the dash out, as well as the entire AC/heater core unit. Bit of a tedious job with all the wires (not actually that many of them).
was stoked to find almost no rust, apart from a small amount on the lip that I showed in photos from the outside earlier, which was the main motivation for pulling it all out in order to be able to access it properly and fix it properly.
most rusty part was the wiper motor mount, luckily no rust transferred from the mount bracket to the car itself so I’ve removed it and will be getting acid dipped sand blasted and repainted, before being reinstalled after the whole engine bay has been painted.