Jay,
You can go to metal store and buy 18gauge sheet metal (mild steel) to practice with. That is about the same as what is on old trucks. See if they have any scraps or odd sizes they would recycle they should give you a deal if they want your business now that you will be welding. May need some odd pieces any way to patch in on your truck. Just thinking if you can find some small pieces you can practice welding them together and not spend time cutting a sheet apart.
When you buy a welder about the first thing you find out is that cutting your pieces to fit and getting that fitment and set up done is where you spend more time than actual welding. I have tried plasma, torch, electric reciprocating saw, cut-off wheel on a rotary air tool, hacksaw, chop saw, metal blade on a circular saw, and a horizontal band saw, among other things, to cut metal.
Probably what I like best for sheet metal is air-powered "body saw", but get extra blades! I have a new air shears I haven't tried yet. But that will be the next thing you start thinking about, cutting, trimming, fitting...
