Read through this build and it will give you plenty of info on the 4th door mod (along with the post and info that Rob linked above) and 1 way to get rid of the high roof crown.
http://67-72chevytrucks.com/vboard/s...d.php?t=306237 I will answer questions for you after you read the build thread and yes their will be a test
But first get the cab back in place and squared up before even thinking about cutting into the 4th door.
Kevin
LFD Inc.