You are really planning a beast!
2000mm long gantry with a dual 200x100x8 steel box section.... I cant really imagine it inside Transit van.
It will be really heavy and hard to build at least in conventional home garage. But it is doable, no questions there...
Dont start buing stuff until 100% finished design!!!
Anyways take a look at http://www.mycncuk.com/threads/6619-...an-Unusual-one
Mister Silyavski is into beasts as well :D
Good luck!