[dsraven]

if you are installing a c notch because you need to drop the rear that much, you may need to do some dimensional checks to ensure you also don't need to raise the bed floor.
if you really want to do it on the cheap without a welder it will take some work. make some cardboard patterns you can lay flat on the outside of the frame. lay it out so you get the axle clearance you need and the height you need, whether you go up into the box floor or not is your decision depending on your wants and needs. you can make a pattern that allows a bolt on part, made out of sheet material, that you can bolt onto the frame before any cutting is done. this ensures the frame stays the same dimensional length and stays true to it's straightness/levelness. you need to X brace the frame before any cutting is done to ensure the frame stays square. use a pilot drill and then a reamer drill to get the tight fit in the drilled holes for some grade 8 bolts that have a non threaded shank where the frame and the new sheet will bolt together. bolt the new sides on, then cut away the parts that need to be cut. use some zip discs to cut the patterns out of sheet and also to cut the frame. once the frame part is cut away you can build the parts you need to connect the dots. this part may require you to build it in parts and have a welder finish the build by welding them together. then bring those parts home and mock them up so you can drill the frame and bolt the whole thing together. when done you can remove the bracing and decide if you want to have the new parts welded to your frame.
the other method requires frame bracing, fab a c notch from rectangular tubing with wings to bolt on to fit your needs and have it welded and ready, then cut the frame, slip the new part in, drill and bolt it on. decide if you want it welded in when done. you should also box the area after.
see, it aint cheap either way.lol.