Just my opinion:

If the project is a Power Plant and the Client specified ASME B31.1 as the Design Basis Code then you must use B31.1 for all analysis.

If the project is a Refinery or Chemical plant and the Client specified ASME B31.3 as the Design Basis then you must use B31.3 for all analysis. The only exception here is if there is a Boiler included in the Refinery or Chemical plant and the Client defines the Boiler piping (only) as B31.1 then only that piping inside the primary isolation valves would be Boiler Code Piping.

You as the Pipe Stress Engineer does not get to pick and choose the "Code" for your analysis that gives you a "Good" answer.

You must use the "Code" that is specified by the Client for his project and do the analysis based on that code. If the configuration passes then that is good. If it fails you do not get to go and look for a different answer using a different Code. To get a problem to pass you must have the piping layout and routing revised to eliminate or mitigate the problem.