Neil, its working now, but incase you cant see it here's a copy and past of the bits i think are relevant to your question..:
The CF Bedford
Part 4: The roof chop
The plan was to send the van to the sand blasters, and finish the rust etc before I chopped the roof but figured since I have to replace the front corner and drivers seat mount pillar section might as well chop the Bedford now.
First up, sanded down the area to be chopped, makes it a lot easier than to sand it down after it's chopped.
Then comes the marking out, I use a marker pen then masking tape over that. Making a template out of panel steel makes life easier when measuring it. Can also use card.
The trick to cutting the front pillars is to make a horizontal line, not a line that is 90 degrees to the pillar as this is not the same height as the rest of the cuts. A lot of people get tricked by this.
The van marked out ready to roof chop
Note the inside pillars are cut at different place to the outside, this is to make a stronger join as butt-welding isn't so strong. The pillars will also have gussets.
Using a hacksaw and cut off saw cut inside the taped lines, leave a couple of millimeters if you can't cut a straight line, then again does a hack saw ever cut a straight line? There are several ways to do this, you can cut all the top lines then clamp a bar on each pillar to hold the roof up off the cut area, then cut the bottom, trail fit, lift again and trim if needed or remove the top section altogether. I guess it depends if you have help or not.
Really there is no reason a roof chop can't be done in a weekend, but in saying that it all depends on the vehicle, amount chopped and number of doors etc.
Once you have test fitted, trimmed and test fitted again, lower the roof to where you think is its final resting place. Now before you weld it, measure all your body lines or factory holes on the inside as ideally you want both sides the same height, also makes for cutting the glass etc easier as you only need one template.
Once you are happy you can start welding your lid back on. If pillars have inner frames or skins, cut the outer skin 50m either side of it and weld the inner section and/or gusset it now as later will be to late. Also try avoid steel on steel as it will sweat and rust later down the track, coat in a zinc primer or similar.
For a more detailed guide to roof chopping a CF Bedford, see the how to guide.
Now off to the sandblasters.
Next - Part 5 - The sand blasting results