First the roof was separated just below the upper rivet strip using a large razor Saw.
Next I scraped and sanded the roof even with the rivet strip.
It was determined that this point the roof was too wide to mate with the RPO body. I calculated the point at which the roof narrows along the arch profile where it would be the correct width. I used my height gage and scribed a line around the roof at this point.
I now had an Arch roof of the correct profile and width to match the car.
The forth step was to shorten the roof to the correct length. A razor saw and miter box were used to cut out a section from the middle of the roof. I then used my milling machine to end mill the roof sections to the correct length to match the RPO body. These were glued together using Testor's model cement, as it has a slow cure time compared to MEK. I used my machined gluing jig to keep things lined up.
The final step was to reinforce the joint from the inside with some sheet styrene.
Next steps will be to detail the roof correct for the 7810-7812 series RPOs.