Firewall rust

Thanks to this awesome How To http://www.bmw2002faq.com/content/view/73/32/ i removed the heater box with not much difficulty

This revealed the worst rust i have found (so far!) 

This is just a big hole between the bottom of the heater box and the transmission tunnel,  not visible from the engine bay because of a rubbery piece of sound insulation with the brake lines running over it

The bottom edge of the heater box hole had also been eaten away but the rest of the plenum looks in good condition, so may get away with a patch here rather than the whole firewall

It looks like i should be fixing this area first as it is hard to get at, so the front subframe/engine/gearbox may come out before the rear