Goodbye, old tunnel! Jeff used a plasma cutter to make short work of the original tunnel, but you can use a die grinder with a cut-off wheel-it just takes a little longer. | 
At this point, you need to lower the body back on the frame, which should still have the engine and trans bolted in place. The 4L60E fits just barely, and Jeff needed to trim some of the floor to make room for the trans pan. (Note the marks scribed on the passenger-side floor.) At this point, you want to step back and think about where you plan to put the trans dipstick and where the shift linkage is going. Trim too little, and you may not have enough room; trim too much and things get goofy in the interior. | 
Jeff fabricated the trans tunnel using three discreet sheet metal forms, all made from 18-gauge cold rolled steel. It's best to start with a sheet of cardboard and build a mock-up of all three pieces first, taping them together and to the body. When you're satisfied with the shape and clearance, disassemble the cardboard pieces and use them for templates to cut the steel sheet. Here, Jeff is rolling the first tunnel form on a bead-rolling machine after shaping the curve on a slip roller. Regularly-spaced beads add rigidity to the tunnel and give it an OEM look. Note how the bead faces down, so as to not interfere with the carpet laying flat. |
Per Mark's instructions, the new tunnel was fabricated with a minimum of clearance. This facilitated the custom console which we'll show you next month. Here's what the tunnel looked like after the first tunnel form was MIG-welded to the floor and firewall. | 
The second tunnel form, which is cylindrical in shape (the first was more conical), closely matches the shape of the trans, which is barrel shaped. When it's done, the whole thing will look like a factory piece, only nicer. | 
The third tunnel form on Mark's Tempest is perhaps a little different than what you'd build, because it doesn't taper all the way down to meet the factory driveshaft tunnel. Because Mark wanted extra driveshaft clearance for the Air Ride suspension, the tunnel was extended to the rear seat base. |
The fourth and last section of the Tempest is for the driveshaft tunnel, which has approximately an inch more radius than the stock tunnel. This gives the driveshaft a bit more room when the Tempest is all the way down on the air bags. It was deemed unnecessary to go back any further, as this would require modification of the rear seats. | 
These are pieces of the original floor cross bracing, which were cut out when the original tunnel was cut out. There are two braces that run laterally under the body to stiffen it, and when these are cut out, some stiffness is lost. On a convertible, this stiffness needs to be restored. Since a rollbar wasn't an option on Mark's convertible, these needed replacing, and that was no easy job. | 
Here, Jeff trial fits the support brace to the new tunnel, checking for a tight fit to the tunnel and the OEM brace-note the temporary scrap steel brace tack welded to the floor. Several of these were in place throughout the tunnel job. The idea is to maintain the integrity of the body shape from beginning to end, especially since the body is being continuously raised and lowered on the frame. |
Here's the completed tunnel with the fabbed cross braces in place. It's a work of art in bare metal. Note how all the surfaces have been carefully ground and prepped for welding. Do a good job here and it will be strong and last forever. Rush through this and you're looking for trouble down the road. | 
After much fabrication, this is what Jeff created to replace the missing tunnel cross brace (two were needed). It's built up from five different metal strips of 1/8-inch cold rolled steel: there are two outer flanges (one on each side), two short vertical channels, and the wide center strap, which is formed over on the ends to meet the remaining sections of the OEM cross brace. They've been hammered out under heat from an oxyacetylene torch in blacksmith fashion (to match a cardboard template), then trial fit to the tunnel repeatedly. All five pieces were then welded together, and ground smooth on the outside. | 
Here's what the tunnel looks like after painting and full vehicle assembly. It looks stock, only better. Note how the cross brace in the foreground looks strong enough to deal with anything Mark can dish out on the road or at the track! |
Jeff Bachar is one of the skilled fabricators at AMC. Jeff not only built the trans tunnel for Mark Vogt's '67 Tempest, he built the entire car. Next month, we'll show you how he built this trick console using the shifter from the 2005 GTO donor car. | | |