I'm having trouble visualizing how shortening the toplink prevents the driveshaft from hitting the deck when the 3ph is raised.
On my tractor, <font color=red>adjusting the upper link length</font color=red> is done by rotating the center barrel. There are two (2) threaded fittings on the ends, one with a right hand thread, the other a left hand thread. If you're familar with a turnbuckle..well, that's what the top link is, basically.
By shortening the upper link, the implement is pivoting about the lower links, thus lowering the <font color=red>front</font color=red> of the implement (and, of course, raising the rear of the implement). This, at least on my mower, results in more clearance for the PTO shaft when the 3ph is raised.
How I adjust my mower is by standing along side the tractor and slowly raising the 3ph until the PTO shaft is very close. Then, I lower the 3ph to adjust the upper link as required.
I may raise and lower two or three times before I get adequate clearance while still ensuring the mower is level.
For your information, the tractor is a Deere 670. The mower is a Befco C-30.
There is an adjustable stop on the rockshaft lever to limit the maximum height the implement can be raised. I'm not sure if this would be called "positional control".
I must set this at a position lower then maximum lift height of the 3ph, otherwise the shaft will hit the deck of the mower.
Hope this helps...