I’ve got the measurement in my build thread. For the life of me I can’t remember it. But it is a straight cut. Keep the same dimension from side to side.
I used a chunk of metal landscape edging attached to the blazer top with a bunch of self tapping screws. I backed off the edging the width of the cutoff wheel we used so the final width was where I needed it.
Order of events went this way for installing it. Set the little section of the blazer top in position. Slide the camper in place, butting it up flush to the fiberglass. Use a couple of long wood screws 1 3/4-2” and from the inside of the front wall of the camper sink the screws in to hold the fiberglass section to the camper.
With the fiberglass attached temporarily now pull the camper back far enough from the cab where you can run the correct size drill bit through the fiberglass at the factory mounting holes.
If you have the front panel finished with paint or some type of skin you can permanently attach the fiberglass to the front wall and seal it. I used screws from the front side to hold the fiberglass firm to the front wall. Those screws need to be just a little shallow than the complete thickness of the wall and the fiberglass if you don’t want them poking through to the interior.
I just used silicone to seal the fiberglass to the front wall. I ran a bead right on the edge prior to installing finally and ran a bead on the exterior after securing it. Keep in mind the middle (horizontal) section is harder to get to with the cab over floor in place. You can get to it but it’s a bit more challenging.
The seals for the fiberglass to the cab are standard top seals for a blazer. One goes on the fiberglass and the other goes in the channel on the cab. The one on the fiberglass is a chore because there are no tabs or clips to hold in place. You glue it on with weatherstrip adhesive aka gorilla snot. You’ll need a dozen spring clamps to hold the sob in place until the glue dries.
Once the weatherstripping is in you can marry the camper back to the truck.
I’ve done it twice now and definitely dialed it on my buddy Ian’s Blazer. On his we never even took the camper completely off the truck. We slid it back about a foot, replaced the wood and skinned it with a white shower board smooth side out and installed the top section in a weekend. It went slick.