body panel welding advice sought

[PARADOX@DEPAUW.EDU]

Hi -

My brother is about to weld in a new floor pan section on a Ford pickup. 
He intends to leave about 3/8 in. overlap when cutting out the diseased
panel in order to give a good surface to weld (MIG) to (from the cab side). 
Is this a good margin?  And how should the bottom side be treated? 
Caulking material? Undercoating?  (It would seem that the leftover lip
would give a splendid place for moisture to gather.)

Any suggestions will be gratefully received.

Thanks,

Louis Smogor
paradox@depauw.edu