Rear floor pan triangles Options

[last337]

I started pulling and poking more and more to get a better assessment of the hell-hole. It appears to be mainly straight down through the fender and through the rear floor pan triangle. My only concern is the condition of the material left on the rear of the long. For the most part the long looks solid enough but has some rust. My friend who will be doing the welding said he can do whatever I need but I am wondering if anyone has welded flat-bar or something along the longs to add stiffness. That may also give the triangle more material to mount to.

[ChrisFoley]

...
but I am wondering if anyone has welded flat-bar or something along the longs to add stiffness...

Please don't do that. Work like that usually has a rather limited lifespan.
We frequently deal with 914s which were previously "repaired" or "reinforced" in a similar manner.
It just makes a proper repair more difficult if not worthless later on, and hurts the long term value of the vehicle.
You don't have to use factory or repro repair panels if you're trying to save money doing the work yourself, but at least use sheet metal of an appropriate thickness which can be shaped so the panel contours are approximately maintained.
If you're looking to reinforce the chassis use one of the three (well - two now) commonly available products: Brad Mayeur Long Kit, Engman Long Kit (nla for now), Restoration Design Clamshells.
The Brad Mayeur kit is more suitable in the case of a car which requires extensive longitudinal repair but the budget doesn't allow for proper repro pieces to be used.
The other two should be used on a solid original or well repaired chassis.