Greg,
I cut the manifold block from a 2" thick block of aluminum I bought at a local metal supply house. It was about 18 bucks. sawed it out using the manifold gasket shown as a pattern. I cut the hole through the block with a 1 1/2" hole saw. The top hole for the tube was also 1 1/2" . Since you are going to use 1" tube, you would only need a 1 1/2 thick piece of aluminum. I welded a short piece of tubing in the block to have something to attach the balance pipe to. for the balance tube itself, I bought two L pipes from a racing supply house. (about 30 bucks for both) the wall thickness is 1/16". To connect the tube to the manifold blocks. Since the manifolds are not square with each other, I avoided precise welding by ordering fuel neck filler hose from the FUEL NECK FILLER HOSE CO.online and using it to connect everything together. (see photos). The bolts that hold the carb boot on will have to be replaced with longer bolts to go through the new block. the outside bolt is about 10mm longer than the inside one. If you search the parts catalogue you can find the existing bolt length and size and in your case, add 37mm +/_ to that length. You can get the metric bolts online from BELMETRICS for about a dollar each. You will also need two 2mm x47mm O rings. You can get them online from THE O RING STORE or maybe from belmetrics also. They are about 35 cents. If you order the same O ring from Rotax, they will be over 6 bucks apiece.
You can leave the existing balance in place if you like. I did at first to make sure everything would work. That way your MAP nipple could still be used. If you do remove it as I did, you can plug the holes with a 1/4 NPT brass plug
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