As the pto joins the implement at roughly the same height as the main lift arms, changing the length of the top link will make little difference to the pto so I don't think this is actually a major consideration.
But Odin has a point, the length of the pto needs to be checked as the machines are not from the same manufacture, and indeed when dealing with old implements there is no way of knowing if the pto has been shortened or lengthened at some point in the past.....it is after all around 50 years old.
To check, lift the implement up and down through its range, observe the pto, when it is dead straight out of the back of the tractor it will be at its shortest, ensure there is no pressure on the shaft (ie that it is not too long)by pressing in the button and checking the pto will pull backwards at least a couple of inches. Next put the mower at its working height , (this might actually be the same position) measure the length of the shaft, then by measuring the individual sections of shaft ensure there is adequate overlap, this is a guess, but a minimum of 6 inches should do it, the longer the better really.
Hope that helps and makes some sense !
You should try and get hold of an operators manual for the mower....a google search should get you one, if not at least buy one for a ferguson finger bar mower, the principles will be the same......some one on here might be able to send you a copy.....ferguson reprints are readily available.