i saw you said you want to do it for a few hundred dollars. That might be a bit impossible unless you get all of your parts for free. Even junkyard parts cost money.
You also have to factor in these small things called tools. People can build these cars/trucks for cheap due to the fact that they have the right tools to fabricate things.
To fabricate parts you will need these tools (ATLEAST):
1. Welder (couple hundred bucks for a good one used)
2. Angle grinders
3. drill press or decent drill
4. cutting tools
5. grinding wheel
that there will cost you around 500-800 bucks and that is buying things used or at Harbor Freight.
I do however know a guy who built a nice little rat rod for 1100 dollars with his own tools, and the help of his friends.