Most galvanized sheet metal (Home Despot type) is crap and will last ayear at best.
Go to a junk yard and get either trunk lid sheet metal or roof sheet metal or at worst the side of a washing machine (hey its coated both sides with anti rust porcelain paint!) and cut out all rust aroundthe holes to clean metal using a Dremel roto tool or a good pair of aircraft snips. Cut new patches 1 in larger than the hole and secure with self tapping screws with a thin coating of epoxy at the joint and lap contact areas. Several coats of undercoating and Bob's your Uncle!

For underbody holes, I prefer to attach sheet metal with self-tapping screws from the outside using a drill and robertson bit. Car on ramps. Then cover everything over with undercoating. It doesn't have to be pretty.

You could patch from the interior but that will mean removing seats and lifting carpet. Also, you'll still have to go under and undercoat.
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1980 245 Canadian B21A with SU carb and M46 trans

You might find Dave Shannon's excellent site useful, here's the rust section: http://www.homestead.com/volvo2/Rust.html
http://www.homestead.com/volvo2/Rust2.html

Erling.
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My 240 Page

I guess it depends on where the holes are and how bad they are. The "right" way to do it would be to order floorpan panels from somewhere and have them welded in, but Yike$$! I would cut the rust out & weld in pieces- junkyard maybe, or maybe your hardware store sells sheetmetal? Then bondo, primer, undercoat. Or you could use rivets, I guess. It depends on what you want to spend, what kind of facilities you have, and of course how picky the inspection station is. Fortunately, up here in Maine, there are still a few garages with a "realistic" attitude. Anyway, good luck!

Matt
1978 244 (Katharine)
1984 244 (Gretchen, RIP)