Did you do the "bucket of water" test? Pour water down the cowl vent and see if it leaks inside. If it does then it's a rusty cowl. If it doesn't, then pour water around the windshield and see if it leaks inside. If it does, then you've got a windshield gasket problem.
here are some pics from when i recently opened the panel on my car: http://mmb.maverick.to/showpost.php?p=565443&postcount=29 might give you some ideas how to start...
The correct way is to drill all the spot welds and fix it from the top as the article says. Others have done different things though - like eliminate the vent or seal it from the bottom.
hey for $500 you can get somebody elses decent floor lol passenger side doesnt look to good http://cgi.ebay.com/ebaymotors/1976...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
and heres a trunk pan for $200 http://cgi.ebay.com/ebaymotors/1977...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories