Steve , the adapter boss slides onto the crankshaft pulley but is a very tight fit even with lube it needs a good tap to get it on the shaft, and even more to get it off and both sets of splines are perfect.
From where i am looking there are several issues going here, none off them are really ideal and all of them are connected.
1) the outer sleeve of the pto shaft appears to be very badly salt water/air coroded and looks to be rusted solid to the adapter boss's splined shaft.
2) The Adapter boss also looks to be in same condition as above and rusted to the crankhaft pulley wheel, bolts could be cut off or ground down in an attempt to get the pto shaft and adapter boss off in one peice.
But, there is a strong possibilty that the spined shaft of the adapter boss may be rusted solid to the crankshaft pulley shaft in the same way.
3) The previous problem may have a direct effect on the 3rd option.
If after cutting/grinding of the 3 bolts the boss cant be shifted, then the crankshaft pulley whel nut cant be removed , thus that cant be removed, leading to a possible replacement of the pulley wheel.
these are prob worse case, but you should still consider the options carefully on this, as things never go acording to plan in this life.
I think its going to be a crossed fingers job here
rob