[MP&C]

The trouble with the rust you're showing is that many times the issues start from inside the roof from condensation and not from water laying in the drip rail. You may not fully see the damage until the roof is opened up. Try an inspection mirror from the inside and perhaps you can determine where the rust originated.

As far as shaving the drip rail, you have at least three layers coming together in some fashion, and likely an inner panel that blocks any hammer and dolly work from the inside. Meaning, any shrinkage that naturally occurs from welding will be there for good. Any additional blow outs from metal still rusty on the inside, and any further weld attempts to fill such blowouts, will cause additional shrinking from the additional heat, likely causing puckers in the roof. If you can't get to the weld seams to planish the welds to stretch out the shrinking effect, it's going to be a mess, no two ways about it.

If repop replacement parts are available and you have no other damage in the cab (crash damage) then perhaps repairing with new parts is a good option. Not likely that new England area has any rust free parts left in a junkyard, but if you're leaning toward cab replacement, I'd look for as rust free as possible in the dry areas of the country. May be some cabs in the classifieds here. See if anything is available, that may help to steer your decision.