[6pack] Attacking bad rust...

[James_]

A common problem with old British cars is rust. 
Well, I have a few small areas that are worse than I thought. A couple small areas ate through the firewall from inside the W/S wiper arm cavern. You know, that in accessible area under the windshield. 
It is on the passenger side near the hood latch. 
Another spot is under the clutch master cylinder in the corner where the inner wheel fender and the pedal box meet. It is fairly small but goes through to the wheel area and through to the inside of the car pedal area. 

Question is...how best to repair this?
Form metal and welding in probably isn't going to work for the intricate curves of the body for the later section mentioned. 

I am thinking of (after grinding/sanding clean) to use fiberglass to give some 'body' to accept the thin layer of body filler to smooth out. 

Other wise, welding in bits of sheet metal would be a pain for many reasons. The car cannot be move right now and I don't know any metal welders that make house calls. Or how they could go about bending/forming metal to even closely match and not look butchered or frankensteined. 

Anyone else deal with this?

Advice?