By David Gensler
I found exhaust tubing suitable for 356 heater tubes in 24 and 36 inch lengths at Pep Boys or Auto Zone (don't remember which now, maybe both, correct 60mm tubing is available from Zim's). They had a variety of sizes. I used 2.250" OD (2.140" ID) tube for the large tubing running through the longitudinals. Interestingly, this tubing was labeled 2.25" ID. It is, but at the end that is bell-ed. I guess this is how they label it. This tube is a pretty good slip fit inside the factory 60mm tubing.
I used a piece from the bell-ed end, slipped inside the 60mm tube and mig welded around the perimeter as a joining piece. Then I fabricated a short piece angle cut, and joined to another short angle cut piece to get the correct mating with the inner longitudinal. I welded that piece to a flat piece of 18 ga. with an oval cutout. Once everything was cut/fit/trimmed, the tube was slipped into the joining piece and mig welded about 3/4 of the way around the joint (couldn't reach the backside). The flat 18 ga flange was then butt welded to the existing inner longitudinal.
The tubing where it passes through the rear frame rails is smaller diameter. I used a piece from the same source that was 2.000" OD (1.920" ID). I fabricated this tube, with its multiple angles, complete from the heater can to where it meets the inner longitudinal. The lower part of the frame rails were cut, and folded back out of the way to allow installation. Like the outer part, the end was welded to a piece of flat 18 ga, which was fitted to the inner long./outer frame rail. This was then fitted, welded in place, and the lower portion of the rear frame rails folded back into place and welded.
I have described this all in reverse, as I made the repairs to the inner, smaller tubing first, then did the larger tubing out in the longitudinal. The Zims tubing will be correct for the outer areas. But if you have to replace the inner portion, you will have to find some smaller tube. I believe the factory OD here is 50mm, so the 2" (50.8mm)OD tube is about 1mm too big. I hit it with a grinding disc and then some 80 grit to reduce the diameter a bit. Then I expanded the heater can flanges slightly. The cans are a snug fit, but go on OK now.
It doesn't look quite like original, since there are 2 welded beads on the outer tube, and the diameter is slightly different. Also, I didn't bother to make the stamped impressions in the frame rail/inner longitudinal where the tubes are welded in place (though I did spot weld the two flat pieces together for strength, as was done by the factory). But of course, no one will ever see it until the next time the outer longitudinals are replaced, and I suppose I will be dead and gone by then.
I do think it is a very strong repair, and I'm convinced that the solid tubes in A cars increases structural rigidity. On the left side, it was necessary to completely replace the inner longitudinal. I removed it and left the heater tube in place for a while. Although the car is secured in a sturdy jig, the heater tube alone was sufficient to tie everything together surprisingly well. Once the tube is removed things get pretty floppy!