This is scary when only condiderations have been made about the modeling of the system but little though about pratical side, fabrication of the spool to exacting tolerances and how to install the spool. How are you going to seat the gasket? Have you looked at the stresses in the flange when you try and pull the vessels togeater to seat the gasket? This is all based on precision during fabrication which does not happen. Even if you had the spool welded up in place for the best fit possiable you would still struggle to get the gasket to seat and give you a leak tight seal with out over stressing the flange and bolts. Your only solution is to tell the truth that they have an unworkable design.