well, here is what i did to my '84 245 a few years ago (i no longer own the car) after buying it from Borton volvo, where they seemed so nice and kind and wouldn't sell you a car where you would fall through the floor, i pulled up the carpet on the driver side only to find a huge 18"X12" hole between the pedals and my seat frame bolt in spots. there was also another large hole behind the driver seat. this is the spot you are talking about.

I dont' know the extent of your hole, but what I did was a temp fix as i didn't realize at the time that a volvo really can last forever and i should have cut out the rust. I placed aluminum sheet metal, cut to fit, over the holes, and sheet metal screwed them in. then used foam in a can to cover the holes, and undercoating underneath the car to keep it from rusting any more that i could.

Needless to say, when I sold the car last fall, nothing looked any different that it had when i put in the metal, cost me about $15 total to do it myself.

once you get under there, undercoat any open metal spots quicky or you will have another hole soon.

good luck
chuck
--
'88 244 174K, '87 BMW 325e 177K (used to feed a '84 245, '84 244, '85 744)