Take off the light and make a template of the back of the light up to the flat ridge that the lens mounts to. You may have to do a little trimming to make the template fit just right. You want to be able to slide the whole light assembly thru the hole and have it rest on the flat ridge. Now you may have to take off all the reflectors and license lightl lenses and such on the sides so you just have a smooth outer shell. You'll probably have to make some covers for the holes that fit inside the light bucket if you don't have the abililty to weld the holes shut. Make sure theres nothing behind where you want to put the light and cut the hole to the size of your new template. Finish the edges to prevent rust. Cut a thin gasket out of stock material to seal the flange to the body and slide the light bucket into the hole you made. You will have to drill a few holes in the lip of the light to secure it to the body, then put the lens back on and your done.
It's easy to write up how to do it but it you pay attention to details and take your time you should be able to do this yourself.
Good Luck and post some pictures when your done.