DVWE Sec IX says almost nothing about type of joint or use. That's other codes, if his client wants SEC IX and they sold that, then they should try to comply.
If this is round, maybe use QW-193, if client wants a Sec IX procedure, then as Ironic said, you must make a weld between samples of a different configuration on proper base metal thickness and weld metal thickness (make this weld between the same .5 x .5 blocks, but make a 1G weld, machine away the excess and test that portion.
I agree with you on path forward though, iam42 should simulate the joint and perform macro-etch tests to ensure it meets requirements. Run as many samples as needed to be confident in your results
If it is force bearing (holding a vacuum), then you may have it physically destructive tested by ASME VIII Div 1, Non-mandatory appendix A.