Figure UW-16.1 Some Acceptable Types of Welded Nozzles and Other Connections to Shells, Heads, etc.
I would also say that you don't need an internal fillet if you do a full pen. weld. Look at figure UW-16.1(c), you are allowed to have internal projection with no fillet weld, and claim the material for area replacement.
Also see UW-16(g)(1) which allows a fillet weld on both the inside and outside with no penetration.
(g) Bolting Pads: Manufacturer’s Standard. The attachment of standard bolting pads shall meet the following requirements:
(1) Except as provided for in (2) and (3), bolting pads shall be attached by a full penetration groove weld or by two fillet or partial penetration welds, one on each face of the vessel wall. The minimum weld dimensions shall be as shown in Figure UW-16.1, sketches (p), (x), , (z), and (aa).