Mock up #1 and #3 exhaust primaries using only 1 1/4" PVC pipe rather than using 1 1/2" for more complex bends.. Using one size pipe simplifies the fitting. 1 1/4" pipe has only two curved fittings: 45 and small radius 90 degree, whereas 1 1/2" has 22.5 degree and large radius 90 degree options. I plan to specify large radius for the 90 degree bends as part of the fabrication instructions. Tweak the #2 and #4 primaries layout to create more clearance between the primaries and the lower cowl.