I'm always put in a 58 cm frame when I go to bike shops. But I think they only initially look at your height. They miss the fact my legs are longer than my younger son that is 6'-3". I'm 5'-11" with about a 35" inseam depending on how you measure. I've always felt better on larger frames. I miss the size of my 26" varsity which equates to a 66cm frame. Ideally I think it was 1 inch to large for me. I probably would have been better on a 25" fram.
And in the 1977 catalog I have for that Varsity, they listed a fit size based on leg length. Here is a copy/paste of
that 1977 catalog page 5.
Frame size Leg Length range*
17" 26" to 30"
19" 28" to 31"
20" 29" to 32"
(53.3 cm) 21" 30" to 33"
(55.8 cm) 22" 31" to 34"
(58.4 cm) 23" 32" to 35"
(60.9 cm) 24" 33" to 36"
(63.5 cm) 25" 34" to 37"
(66.0 cm) 26" 35" to 38"
I added the metric conversion to the frame size that was given in inches then. As well I realize that bike geometry has changed somewhat since back then, so todays 60 cm frame might have tubes lengths and angles that make it fit like the sizes of bikes in my youth.
But I believe that bike sizing today is still has the same issue as then. Different people have different arm, leg and body lengths. But the sizing systems were built toward an assumed average. As well the calculators that supposedly take those measures and more into account base their size on a particular geometry or two which might not be what you are looking at in the store.
So trying a size or two in both directions seems rational and prudent to me. And as you seem to be into very old bikes too, then today's frame sizing might not work either.