Finally I needed to get the bends at the tail end of the tubes. These are really tight bends - far to tight for the tube bender since it needs like 4" of straight tail, so my best option was to bend up a the tubes to the angle I wanted, cut them and weld them back together to keep it compact. I also knew that since I'm doing a K20 swap which takes 1 1/4" coolant tubes, I might as well choke down just after the tunnel.
These need to be leak-free, so in addition to pressure testing which I did later, I picked up one of those cheap USB cameras you hook to your phone and investigated my welds on the backside. Looks good.
Now I needed to cover the whole lot. I picked up a piece of SS sheet and had a local metal fab shop bend it with their giant bender. This was drilled for the specific locations on the tunnel spars and fastened with some M6 SS bolts (yeah, went stainless crazy where I could).
Finally, to keep it relatively clean (and mostly dry) in the tunnel, I needed a way to cover the front. While the rear structural support already works to keep water debris out as it is at the very end of the tunnel, the front doesn't have that since the spar and tunnel cover moves forward past where the return line for heater drops thru the tunnel floor:
I fabricated a simple 3/8" aluminum clamp/cover that bolts over the exiting tubes and is perfect size to stay with the tubes and block the tunnel entrance. This is not structural but looks nice and complements the SS.
That front plate will get some kind of grommet to go around it to further keep the tunnel area clean of as much water as possible. The structural members or anywhere else on the tunnel cover won't get any kind of gasket treatment so any water that makes its way in the tunnel can flow out towards the back.
I did ALL of this because I wanted the chassis to be stronger. Tubes are by nature very very strong torsionally and since I've got two of them, why not make use of it. Any twist the tunnel wants to do will force the clamps to twist (due to the strong vertical spars) and since the tubes are welded to half of the clamps, it will transfer that torsional load forward to the next clamp (midway and vice-versa). It'd be even stronger to use steel tubes and just weld them in place, but then they wouldn't be removable.
This was fun, but not practical.