I believe SA387-B is now SA387 Gr. 12 (1Cr-.5Mo), which is a P-No. 4 Gr. 1. The correct electrode would be E8018-B2 or E7018-B2L. Preheat should be 250 F minimum. Postweld heat treatment is 1200-1300 F.

I'm not sure what you mean by "dismantling". Is this for cutting the vessel apart for disposal or is it for disassembly for periodic inspection/maintenance? If it is the latter, P-No. 4 materials are typically used for elevated temperature service over 750 F, and you need to be aware that carbon steel may not be an appropriate choice for service at that temperature. Graphitization can occur in carbon steels at those temperatures, usually in the weld HAZ first. This could lead to the lifting lug failing when you use it. If this vessel will continue to be used, I hope the appropriate engineering organization has evaluated the lifting lug design and material selection, and that the appropriate ASME or National Board Certificates of Authorization are held by the organization doing the welding.