The engine is going into my '57 3600.
The manifold is made from two manifolds, a good one and a donor. It's a shadetree mechanic's way to replicate the '53-'54 Corvette manifold. The good manifold has a 1/4" slot cut into the back ahead of the #4 cylinder. A plug is carved with a die grinder to fit inside the slot to block off the front cylinder bank from the back bank. The plug is welded into place when the slot is welded shut.
The exhaust dump off the donor manifold is cut off using an angle grinder. The heat flapper box is cut off at an angle. Fabricate a piece and weld it to the exhaust dump where the heat flapper box was cut at an angle. The exhaust dump is then is carefully ground and shaped to fit over the 4-5-6 cylinder end of the manifold about where the part number is cast. Once the dump fits correctly, the outline is traced on to the good manifold. Move the lines in 1/4" and cut out the shape. Bevel the edges of the exhaust dump to accept weld, then weld into position.
All of the fabricated parts are made from metal from the donor manifold This avoids mixing metals which can create welding complications and expansion/contraction issues during operation.
Welding cast can be tricky to prevent cracking and warping. I used a #6 rosebud torch to preheat. I mig welded with Crown Alloys Royal 44-30 .35 wire which is a nickel manganese blend designed for cast iron. I welded in short stitches of about 1" and used the pointed end of a slag hammer to relax the weld after each stitch with about a minute of steady, firm tapping. The first pass was for penetration. Additional passes were made to fill. Once finished, the weld was buried in sand overnight to cool slowly. It was ground down after it cooled off.
Here are a few more pix including a couple that I pulled off eBay about a year ago showing the front and back of the Corvette manifold. Note that the Corvette manifold is cast so that they cylinder banks are separated.
http://s844.photobucket.com/user/ma...2040_zpsghebo01x.jpg.html?sort=3&o=2I bolted the intake and exhaust manifolds together and had the machine shop mill the flanges perfectly flat while they were joined together to get a good seal just in case there was any warping caused by the welding. That cost $35, but it was well worth it.
I went with this design to get the benefit of the split exhaust and still retain the factory heat exchanger system.
Matt
