Welding Tips and Tricks

I'm building out several large steel containers to go on top of a few trailers to act as mobile dumpsters, and will need my 1 Ton gantry crane (pic below) to lift the containers (about 1,000 lbs each) onto the trailers once built and to then do the final welding in place. Problem is that one of the trailers is 6 inches too wide due to the wheels to get in between the gantry for loading on the container.

My thought here is to cut one side and put an extension (18-24") in the I-Beam that serves as the top support of the gantry. Problem here is that it appears to be a metric beam (120mm x 75mm) and I can't find any of that locally or from reputable sellers online.

Any thoughts on making an extension like that without using an I-Beam? Is it way out of line to just weld thick stiffener plates on each surface of the beam to serve as the extension? It's only 1 ton rated as is.

Thanks for any help.

Tony, I would get a new beam of greater width and not cut the original. Ymmv.

(edit)
Might also consider a forklift rental. I got one here at work for a day for <$100 delivered. And it was a big mutha, 15K#.

Can you load it over the back of the trailer, getting the gantry up as close to the wheels as possible. Re-attach to the rear of the container, and lift it to get the weight off, then slide it foward into position? Maybe a few pipe rollers between the container and the trailer?

Tony, as buggy stated, rent a large fork truck, replace the top rail with one of our beams or instead of cutting the gantry, can you come up with some type of jacks ?

Thanks for all the ideas! Definitely better than what I was leaning towards. Might even just try the pipe rollers as mentioned and a come-along to pull it forward since it is a one-off thing. I'll update with how it goes.

Thanks again.