Yes, the 3912600 was the stock manifold for the 4bt with the P7100 and an HX30W turbo. Also came on engines with the H1C turbo. Probably about 3/4 of all the road 4bts had that manifold. The rest had either the top center mount version or one similar to yours only with a compound tilt 3906661. That one only came on CPL767 and Cummins discontinued it and changed to the one you have now. Only issue is finding the 3912600. Used to be as common as dirt but the one you have and the center version seems to be all we see. The price of a new one gets into the $300 range where the others can be found under $100 new. As far as down pipes, here's a photo of one on my workbench but don't know if it would clear your firewall even with the turbo moved over. This one has a section of 3" pipe attached.
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