Ok ladies and gents, to start with premanufactured floor pans aint cheap. Just google it and you will find a couple of different suppliers. Classic to current is just one company, Sherman is another, like i said Google. If you have to replace both sides you lookin at 200 to 250 ship included. BUT if you have a lot of determination and a lot more cutoff/grinding wheels you can make your own. WARNING: THis is NOT easy, but it can be done. I just finished doing mine this way about 2hrs ago, seriously. I used 3/16 cold roll sheetmetal, WAAY overkill, but it worked out nicely. I picked up all that i needed and then some from a scrap yard for 15 bucks. The decision is yours to make, if you have the money go with the premanufactured.
As for dealing with spot welds get a QUALITY spot weld cutter, Blair to be exact. I was stupid and paid 50 freaking dollars for their "heavy duty" model, ya it worked great for about 25-28 welds and wont break on you, but it DEFINALETLY wasnt worth the price. Especially since replacement heads cost just as much. It wasnt until i was half way done that i found a local Auto Body supplier that carried their basic cutter that i realized it was all i needed. It was $45 for Blair's basic tool, it came with a 2 sided bit, and 3 replacement heads. 4 heads = 8 cutting sides = approx 260 welds, JUST BE CAREFUL these spot weld cutters work really REALLY well. The only downside is they break extremely easily, so dont tilt you drill to the side or get it in a bind, just push straight down with little to moderate pressure. If your drill has a low torque setting use it and go at about HALF throttle. This is what i found works best.
For finding spot welds, ya, it can be pretty frustrating when the metal is completed rusted over and you have no way to distinguish what is and what isnt a weld. The only thing i can tell you for this is to just drill out the ones you can see clearly. Once that is done pry up on the metal until you can see where it is being held down next, this is the next spot weld needed to be drilled out. And if all else fails, cut out the metal above the frame rail, LEAVEING the spotwelded strip ontop, use a grinding wheel to basically grind away the strip until you get to the frame rail below.
For coating purposes I have heard great things about POR-15, Google it. I used a liberal amount of some stuff called Permatex: Rust Treatment, followed by a two coats of black tractor paint. The Permatex can be found at your Auto Parts store and the Tractor paint at TCS, Lowes, Howe depot, etc.
When the time comes to start welding your new floor pans into place, i would recommend using .023 wire, 75/25 gas mix, and somebody who has at least some experience in mig welding, as "blow through" will be your enemy #1.
I would post some pictures of my project, but the computer i use to do that is in the "shop", unfortuatelly that is one of the few things i cant fix.
hope this helps fellas