I am in the midle of welding in patch panels in my '66 C10 myself...First time I've done this. My experience so far is that its much more complicated than it appears...Between rust and trying to find and drill out all the spot welds, its way more time comnsuming than I thought it would be, not as easy as cut out a piece and weld in the new one. The other thing (and that part that is relevant to your question!) I've learned is that patch panels for the most part are a starting point...They are a rough approximation of the part they are supposed to replace and need at least some cutting, bending and modification to do the job. Still much easier (usually) than starting form a flat piece of sheet metal, but definitely not a plug and play proposition.
That said, its very satisfying seeing a floor where there was just a rusty hole, and knowing I did it myself...