Well, maybe not. These cars were designed in the days before finite element analysis and validation crash testing. Also, given their age, it is likely the spot welds are weakened from low-cycle fatigue. The body is as likely to be much weaker than originally designed as it is to become too stiff from seam welding....Seam welding on a street car is a very bad idea: it can prevent the body from deforming in the way it was designed to in the event of a collision, increasing the risk of injury to occupants....
In addition, there are a number of areas that were noticeably weak in the early ('64-66 Alfas) that the company improved in later models. The front cross member was made of heavier gauge steel in later years, as were most of the front inner panels. Alfa even provided details on how to replace the early (two-bolt) cross member with a later version (four-bolt) that includes fully seam welding the new unit.
And we all know of the weakness in the steering box (and idler arm) mounting that has lead to many reinforcements.
Given that Alfa even recommends reinforcement in the front areas, I'd happily go ahead if you are confident in your skills. If you do the seams in short segments, you will not have any warping issues.