like em says! take the roof of the donor van so there's nothing in the way for measuring. then you can take your time to get the measurements right, check check check and again, once you've marked the hole on the roof of the van, hold the steel frame off the donor up to the marks, check for clearance, make sure it's in the right place, and that you are happy it's centralized and where it needs to be.
when you are cutting, straight by cutting the hold a good 2" smaller than you need. this way you can then stand in the middle of the hole and cut more accurately with out leaning over and risking a "cock" up. Make sure you have plenty of jigsaw blades if that's how you are cutting, but if you can get one use a pneumatic nibbler as it will make a lot less mess. Any little bits of metal that land on the body work need to be removed immediately, as those that are not can end up causing rust all over the place.
once the hole is cut and trimmed to size, start fitting your components, frame first, then bed supports IIRC, you might want to take this time to renew some of the parts or put new fixings in them to keep them rigid.
One piece of kit that is well worth having, hiring or borrowing it a pneumatic riveter! will be alot quicker and save your hands arms and wrists from much pain with all those rivits to put in. one pull of the trigger and the rivet has popped, instead of trying to do them all by hand!
Good luck, and lots of pictures please!
