All MetalShaping - Hi I have just started ratting a 37 plymouth truck

Hi,
i picked up a 37 plymouth truck a couple of years ago and have just started to rat it. The body was in real good original shape (never painted).I have installed a chevy s10 rear end (same width as original) The rear spring shackles have been flipped to lower it and i removed 3 springs.The frame has been blasted and coated. I have a 1958 392 hemi with a 727 tranny to install. i just bought a set (4) tires and wheels the wheels are mickeys and tires are hoosier`s. The tires for the back are 19 inches wide and i am wanting to widen the rear fenders but im not sure where to cut or start for that matter.They are in really good shape,so I don`t want to ruin them by not knowing what im doing.
Steve