"Sure, test fitting sounds like twice the work. But in today's world of reproduction parts, if all of the manufacturers are to the edge of their tolerances, without dry building before paint, you may find yoruself shimming, cutting, grinding, and even slotting holes to get the proper fit," Jim says as a note of caution.
There are several aspects to this one, from checking fitment on a new chassis going underneath your car to making sure you're A/C unit fits correctly underneath the dash. In the case of the '70 convertible, the Vintage Air climate unit requires two tabs be secured to the firewall for proper mounting. To make sure the unit would be nice and secure, the shop went ahead and welded them to the firewall—something that wouldn't have been possible if the body had already been painted. Then there were the modifications to the firewall for running the A/C and heater lines, requiring further mods to the firewall.
Both of our project cars have had full aftermarket chassis installed in place of the factory originals. For the Chevelle, we had the prototype Art Morrison Enterprises '68-72 A-body chassis. One of the newest pieces out there, the AME unit bolted to the body a bit differently from the factory frame, require some test-fitting and tweaking to get everything just right. Our Fatman Fabrications chassis underneath the '55 has been in production for a few years, so the necessary mods for fitment are listed in the instructions. It still required some small mods for an exact, proper fit, but they were a known quantity.
Checking drivetrain fitment during the dry-build process requires the engine/trans to be installed and removed several times on a custom build. On a painted car, you only want to be installing the engine once, because each time it comes out increases the odds the paint will get nicked. This process also lets you check firewall clearance with the engine. Even though the Chevelle came with a big-block from the factory in '70, throw in our aftermarket chassis that tucks the body in closer to the frame, and you could have issues that require cutting and welding on the firewall.