Wheel Diameter Help.
04-27-19, 08:58 AM
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The confusion has existed from the beginning because the tire manufacturers insist on using the outside diameter measurement of the tire, which may or may not actually match the tire in question. Vehicles, by contrast, use the dimension of the rim. Fortunately someone along the way started adding the ISO/Bead Seat Diameter to the tire marking, which makes things slightly less confusing. Now, about those 26" tires on your 60's department store bike... 
regarding fork sizing... in general the difference in radius between 27” and 700c is 4 mm about, so, the brake reach all things being equal would require that much longer for the 700c rim.
beyond that, its a bit era dependent, from the 60’s to the 80’s many mfgs reduced clearances, so the “700c” vs 27” fork length is a moving target.
in general, one wants to have the bike sit on its “ lines” top tube level, head angle as designed and matching fork rake. Minor variations will still work, a longer fork crown race to axle length ( not measured direct, but think framing square, which can provide rake and length) will slacken the head angle a fraction. In most cases tolerable.
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