Welcome, this is as good a place as any to post about your lightweight.
Rust Treatment of bulkheads is the same as any mild steel.
Either use zinc. Or paint. Or both.
I've a galv dipped bulkhead for my lightweight, but as the hot dip process caused it to ripple I'll spend a lot of time straightening it out. This will degrade the zinc, so I'll get it hot zinc sprayed and then coated in 2 part epoxy paint.
Top coat will be NATO green in satin.
Now, as to chassis. How bad is it?
Take a look at my lightweight thread on the members vehicle sub forum, is it as bad as that?
If you get a half chassis you'll have to ensure its for a lightweight, if you want to keep it original, as the lightweight chassis can be quite different to a standard series, in particular they have a military rear crossmember, extra bracing, a different seat belt mid crossmember and extra riggers for fuel tanks.
I don't think you'll get one, so you'll have to modify one, by which time it might be easier repair what you got.
Post up some photos, let's have a look!