I hate to say it but that sounds like an accident waiting to happen. It's one thing to do things with "cost-effective" in mind, but to try and be cheap will cost more in the long run, especially if he is going to be "mixing and matching" floor pans. The floor pans are an integral part of the torsional rigidity, and if there are a bunch of cuts and welds and patches, that is not a good thing. The body panels can be doctored up enough to make them look presentable, but the floor pan is not something you want to be half-arsing it on. Spend the money to buy a new floor pan is my advice. It will also save a ton of time.