I haven't dome the conversion myself, but I've researched it.
Apparently, the holes for the assemblies are different where they mount, on the inside of the van.
Our later model vans have smaller holes, or there are mounting tabs that would block the older style assembly.
So, to mount the older style tail lights, you have to remove some of that metal, so that the older assembly fits, then devise a way to retain the assembly in that resized hole (I've seen people just use a flat piece of metal to act as a cross brace).
So, there is some "cutting and hacking", but it's not very severe, and is not on the outside of the van.
Some of the guys here have done it, and there's a thread about it somewhere around here.
Maybe one of them will chime in.