I have replaced both pans in my 80. It's a biggggg job. If you decide to do it I recommend this http://www.harborfreight.com/high-speed-metal-saw-91753.html and buy some extra blades.
My original pans were metal from the factory and I am not sure about yours. Take a lot of measurements before you cut. The pans you buy will also need to be trimmed. On my car I cut out the old ones, fitted the new ones and then temporarily held them in place with a few screws until I could weld them in place.
When I was finished I glassed in all the seams to prevent any water from coming in. I spent about 2 days per side doing the job. Cut a smaller section out of the old floor and start from there fitting the new one in.
If you cut too much then you will have some large gaps to fill and strengthen. This is doable from the top side of the car. I purchased my pans from MAM but other vendors have them also. The metal pans are heavy and bulky to maneuver around in the car sometimes.
Take your time, take breaks, take Motrin, and do a good job. Dennis B.
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Corvettes and women are the same...You want to make love to them but they keep breaking down.