Longitudinals repair, how bad before I throw up my hands? Options

[pek771]

I'll post some new pictures when I get home. I may have the record for rusted longitudinals, but I am going to fix them anyway. PO did a lot of shoddy quiltwork patching, and when I got the car opened up really well, it is ugly. I got the heater tube out of the drivers side long just a minute ago. The amount of rust is mind-boggling. At least the car is lighter now. It has taken three weeks to get it cut up this far (link in my signature)

How did Porsche construct the longs originally? Was it like a shoebox turned on its side, with the lid facing out (the inner rocker panel is the lid)? I can make a new box of 16 ga steel, roughly 6-1/2" x 4" with some internal stiffeners, and bulkhead the rear to help carry the suspension. Weld the new box into what is left of the existing longitudinal, put some new inner rockers in and be done with it.

This sounds too simple, even though I have a sheet metal shop and my MIG machine at my disposal. Anyone ever try to do this before, or am I venturing into new uncharted waters? Or, am I delusional?

Thanks!
Pat