Well my 2 cents worth:
Based on working on a yacht of almost identical age and similar design..
I do not belive this is any problem you should be too scared from. The boat is almost certainly built in two halves and the "crack" you can see is a void in the Gelcoat
. The strength in the hull comes from the lamination on the inside and since this is a late 60 ies production designed by the masters at SS they sure have dimensioned it like a tank ( they did in them days).
So where does the "crack" (void) come from?
First , the two hull halves were clamped together and then glassed with many layers of glass ( as thick or even thicker than the hull) to join them into one strong hull. This was sure done before the polyester had fully hardened and thus ensuring a perfect chemical bond of the laminates, ( within some 4 weeks depending on temperature)
So the important thing here is the inside lamination and when this was done you should take care of the cosmetic filling of the gelcoat
, This undertaking is for sure not just in this "birth void" of the boat but in many other places where the gelcoat is not perfect from comming out of the mould.
I belive that in filling this crack the person doing this missed getting the gelcoat fully into the void.
I belive they intended the yacht to have a metal rail in the bow from the top down to a bit below the waterline. In this case the " crack " would not be visible.
If this would be the result of some overstressing the laminate due grounding etc the thing would look entierly different with cracks running perpendicular to the stress or "star shaped" cracks (very fine) due sharp pointed impact.
In a hull looking like that one I would be looking for stress cracks 10 feet down the freeboard from the bow since the hull do not have much curvature there and need supporting or strengthening to take care of hard beating in heavy seas.
I belive she is a boat really flying upwind ..
the hullform and the dimension standards when the boat was built gives that you do not have to think about the strenght of the hull.
I would be looking at other things as the rigging
( including chainplates etc) , Steering
and the whole electrical system
not to mention tanks
( both water and diesel
to determin how much money
must be spent to be sure you have a reliable cruiser.
I am sure you would get a fantastic yacht if the total survey by a reputable surveyor indicates a yacht in a condition that you expect for the money
you want to pay including the money you are to invest in things ageing to the point of needing to be fixed.