I bought the 'correct' sheetmetal (a story itself) and had
a neighbor weld it in to replace my rusted-out battery support.
I now believe this was far too anal-retentive on my part. Actually
replacing the battery shelf in 1953-1979 models requires actual 
professional body repair skills my neighbor didn't have...
	If you've  got rust damage, fix that- cut away what's rusted 
out, clean down to shiny metal, prime and paint. If this leaves a hole, 
I'd pop-rivet or bolt (with nylon locking nuts) a filler plate made of
painted steel or aluminum over the hole. Use brushable seam sealer,
galvanized gutter sealer or RTV to seal everything water tight
	OR buy the repair part and cut a piece out of it to repair
what you need, bolt, rivet or weld in place.
	OR find someone who is competent, or learn yourself, and
do the repair right with the repair part.

	I, personally, wouldn't use plywood. If you do, I recomend you
prime and paint it with oil based paint for wood, use big washers
and stainless steel hardware.

	Cheers!
	Bill