NOS drop in floor panels are most likely extinct and I wouldn't hold my breath for any really good salvage cars. They all seem to rust in the same areas but in different amounts.
You might check out Classic 2 Current
http://www.c2cfabrication.com and see if they have what you need. Their panels are not perfect but are a starting point and will need some nips and tucks to make fit. Probably the only thing around short of the extreme luck in finding a parts car with better metal than what you have.
Several on the forum have used C2Cs panels and if you do a search you will probably find some blogs. BigKev used them on his 54 and IIRC there are some photos showing what he had to do.
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