Dodge D5N LWB tray and RHS passenger door

Hi all, I bought a 74 Dodge D5N dual cab Variety Bash beastie and have repainted it into Navy colours. I am led to believe that it was an old State Rail Crew vehicle and has the steel tray and very large, what would have been tool boxes I suspect, mounted on the sides. The RHS passenger door does not open, nor does the window wind down - looks like it is sealed and I notice that on a few that I have seen on the web - something about safety so that passengers do not step out into traffic.
I find it a bit bulky and certainly very heavy and would like to redo the back end after this bash as well as make the RHS passenger operative.
I would like to take the bulky and heavy tool boxes off, possibly cut off the extension and replace it with a Ute style tray.
Has anybody done something like this before who could advise me on the best way to go. It has a 300+ litre LPG tank as well as 2 x 100+ liter unleaded tanks.
I have another back door answas wondering how hard it might be to replace.

Cheers, Adrian