JEBalles,

As you know Ive just done this type of stuff. My advice would be to clear out the loose rust, POR-15 and then use epoxy putty. You also have your fuel lines in that area which are not helpful when welding.

The technique is:
1.) Coat with POR-15.
2.) Pain in fiberglass sheets with POR-15 to create form of sorts
3.) Put in the EPoxy putty
4.) Overcoat with POR-15's chasis coat black or similar.

I have the chassis coat black and the fiberglass for when the time comes.

The caveat is that you will not know how bad your issues are until you remove all the undercoat and the sound deadener to see the entire cancerous region. It could be really horrible.

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car