Make sure you weld some sort of support in to prevent the front of the cab from creeping backwards as you cut all the old metal out. I can't stress this enough !!If you don't do this, your doors will never line up again.
If it was me, I would reinstall the doors before proceeding forward to make sure things haven't moved already. A bit of work now could save you a lot of grief later.
Dan
EDIT;
Take a look at the build tjsblazer did, and you'll see what I mean by the reinforcement I'm talking about. He used the original hinge mount and door latch area to bolt his on, so it didn't have to be welded in.
http://67-72chevytrucks.com/vboard/s...d.php?t=626752