I'm not that familiar with the ASME VIII stamping rules, but what you're describing sounds like the customer will supply ASME materials which you will weld under your quality program using your welders and your qualified welding procedure specifications. And perhaps your inspectors will perform the NDE. If this is the case, and you will be using your ASME Certificate of Authorization to perform the work then you would need to give the customer some sort of partial data report, even though the sub-assy itself may not require stamping. I'll have to do some further reading of VIII to see what's required in the way of stamping and data reports.