Manufacture of a gas turbine exhaust diffuser shell (40A/40B) to achieve a final cross-sectional shell geometry by forming an opening (76) in the shell to receive a strut shield collar (46); forming a compensating outward bowing (78) of the shell around the opening that departs from a desired final shell geometry in an amount and shape that compensates for a welding shrinkage when welding the collar in the opening; and welding the collar in the opening. This produces the desired shell geometry after the welding. The collar may be welded proximate an edge (74) of the diffuser shell, such as along an intersection of an axial plane with the diffuser shell. A multi-bolt flange (68) may be welded to or otherwise formed along this edge for assembling an annular exhaust diffuser duct (38A-B, 40A/B) in an exhaust section (20) of a gas turbine engine.