I am pretty sure that the top link for a JD3150 does not use the std ball swivel at the tractor end. The knuckle end (u joint) type end hydraulic top links that are available off the shelf sometimes do not fit without reworking them. So that $340 kit cost + shipping of a heavy unit is most likely not going to be his total cost. And then he needs to get one that is the right working length, that can be another problem.
So for the OP, a properly fit hydraulic top link is not quite so easy.
I happen to know of a place that can take care of him though, just needs to be patient and have a few extra $$$ if he wants a unit that fits right.