if you want it done right and to last for the next decade, Sirmebes is right, go to a body shop. but if you want a cheap alternative, here is what i did on my '84 245:

i pulled the carpet up over the affected area's. roughly measured the size of the holes (2 large ones i had) found some aluminum sheet metal cut it to fit over the holes and screwed them in. (i was careful to not drill through the fuel lines, and make sure you pre-drill the holes before you use screws)

next, i sealed the diverted edges underneath with the hardening foam in a can (insulation type stuff), let it dry for a 1/2 then sprayed undercoating over everything. it looks the same today as when i did it a year ago.

Hope this helps,
Chuck
'84 245 216K
'88 244 154K
'85 744 175K (this is for sale, hint hint)