Chopping 'glass is a bit more in the realm of reality for the at-home hobbyist than cuttin' a steel body would be. The process is the same, but the reassembly is much more forgiving because minor errors can be corrected using 'glass and resin versus metalwork and welding.
In my humble opinion, there's nothin' like a heavily hammered A sedan. A belly-high roofline just looks right, so when I had the opportunity to witness a chop job on a fiberglass A sedan body, I jumped at the chance. Recently, young Nate Pierce of Bruiser Enterprises in Lake Havasu City, Arizona, was performing the task on one of the company's way-cool Bruiser "Finish It Yourself" '28/29 sedan bodies, and he was kind enough to give us a few pointers on the task as he went along.
Now, the best thing about lowering the lid on a 'glass body is that it can be accomplished without welding gear or equipment-being 'glass, all that's needed to sew a body back together is a bit of bracing and a bunch of resin, cloth, and matting. In this case, we have to keep in mind that the subject shown here is a bare-bones Bruiser sedan. Bare bones meaning that it's an entry level-priced, un-reinforced shell that makes this chore that much easier, as the reinforcement is added to the body after the lid is at its desired height. This is something I plan on doing with mine in the near future. That said, let's get on to the photos and see this chop job in action.
The first step in any chop job is deciding how much to whack it. In this respect, it's pretty much a matter of taste, if not practicality. Nate considers a 5-inch chop to be ideal for an A sedan-this gives a good look and still leaves enough in the way of headroom for the average rodder. Another thing I should mention is that the Model A is a perfect candidate for the first-timer; the boxy shape allows the lid to be dropped without the need to stretch the top to align the pillars. | 
The easiest way to map out the chop is to use masking tape. You'll want to take out your body material from the straightest, most vertical sections of the body, which in the case of an A is near midway through the window openings. A die-grinder or Sawsall can be used to make your cuts and work just as well on 'glass as steel. | 
Your first set of cuts should be made at the uppermost cut line (as shown in the previous image) this will allow the removal of the roof section as one piece and lets you proceed around the car section by section, removing the remaining pillar pieces one at a time. |
With all the sections (all 5 inches in height in this case) removed, you're then almost ready to lift the roof section back into place onto the lower body section. | 
Here you can see a close-up of one of the B-pillars that have been prepped both inside and out for re-'glassing. For those unsure of what B-pillars are, they're the second vertical pillar rearward of the windshield or A-pillars. So, a two-door will have a pair of A-pillars (at the windshield), B-pillars (at the rear of the doors), and C-pillars (at the quarter-panel to the rear window). | 
The next step is to lower the roof section back onto the lower body. If you've made your cuts as precisely as Nate has here, the roof panel should fit the body as well as it does in this case, and everything should line up pretty darn closely. You'll then want to clamp the upper and lower sections in place. Nate used welding clamps and pieces of plate steel placed at all four corners and at the C-pillars to make sure the two pieces align precisely. |
Before you do that though, you'll want to prepare the remaining sections of 'glass for reassembly by roughing up the edges to give the new resin and fiberglass a good mechanical bonding area (this should be done on both the inner and outer surfaces of both the upper and lower body sections). | 
Here's a shot of the righthand B-pillar. You can see just how well the upper and lower panels line up if you look closely at the cut line-this is what you want all the way around. | 
The ideal place to start the reattachment process is on the inside of the body at the largest contact area-this will make for the strongest bond (because of the large area covered). Also, once the resin and mat has set, it'll hold the top reliably in place for the balance of your work. |