The 260 engine, in and of itself, in a Mustang, is not really that rare. I believe about 45% or so of the population had it. When counted against the entire population of 64.5/65/66, then it is unusual, and some parts are hard to find. However, it was also in a slew of Falcons, Fairlanes, etc, so, if you open up and check out those cars, parts become pretty easy. Case in point: 64.5, either 260 or 289 used a 4 groove cast iron crankshaft pulley if it had AC and PS. 65/66 used a 3 groove stamped steel. I would trade away all my 4 groove cast iron cranks for 3 groove steels, because the 3 groove outsells them by about a 4x price difference. Lots of supply, little demand.
As far as the toploader goes, rare yes, unusual, maybe so, desireable: NO. A 5 bolt toploader will only fit a 5 bolt bell housing, which will only fit a 5 bolt (221/260/early 289) engine, vs a 6 bolt toploader which has both patterns on it, will fit from 1962 up. Also, the early toploader had a 25 spline input, hard to find a yoke for it, plus the cluster was weak. Ford upgraded to a 28 spline yoke and a stronger cluster. I have passed on several $100 - $150 25 spline toploaders due to their limited use and serviceability.