A 60-66 frame should fit a 66 cab and front sheet metal a lot better than the modern frame.

In general if you can't think of any advantages to changing something then leave it alone.
With that in mind, for what reasons were you considering the 73-91 frame?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-