Drifting down?  You just do not have the hydraulic lever in the "hold" position.  There is a sweet spot in the middle of the quadrant where it keeps its position.  I've never had a problem.  I use a John Deere 5' 3 point.  Use the hand clutch if it starts to struggle in tall grass.

I do not believe in "torching" something off to make it work better.  It was made that way for a reason.

Limiter chains are a close second to a AC 160, or 172 bush hog, with the "rooster comb" height adjuster.  Best bush hog height control system ever built, bar none!  It was a chain that connected each lower 3 point pin, to a bent bar, that was hung over a notched top link, to adjust height, you just pick up the BH, then reach back, adjust the bar, drop the BH, and go!

If you want a better chopping action, the rear should be lower than the front. Level is what it's "supposed" to be, but I generally adjust my top link so the rear is maybe 2-3" higher than the front. Supposed to use less power that way. 

Look at the rooster comb piece, in place of the top-link...

Need to get her where she is pitched with the front doing the cutting, otherwise you are just keeping everything you cut under the deckwhich uses more power and fuel, and throwing clumps for cuttings.

5' Cutter will not cover the PAW's adjusted all the way in on a 15 I/II, just reverse direction to catch the previous pass with a small overlap to get the brush/grass you laid down on the last pass with your tire.

Make sure your blades swing freely and retainer lock nut is tight but not too tight to prevent the swing effect. With cutter all the up and a good jack stand on each side, a grinder can be used to "touch up" the cutting edge on each blade to make a substantial difference in cutting. If any major "dings" in blade cutting edges, polish off the excess vs. trying to make a new edge by re-forming the blade back to it's original shape as removing too much material will throw you off balance while running.

Looks like that unit needs a chain guard or belting on the front, not good to run without one.

Shear bolt in driveline AND slip clutch at tractor pto will keep from pushing you around and if you think you can always throw the PD into neutral in time, it will eventually get you. Good brakes are a must as well.

Have a 5' A-C with a good size gear box but picked up a Bush Hog SQ72 (6') some years back and it is well matched for both the SI/SII units. I have no issue with cutting stuff above the radiator cap and take out trees 2" OD with a little practice.

I warm her up good before tackling the rough stuff, getting the hyd. fluid good and hot.

Go slow, 1st high or 2nd low on a D15 of either series for a decent cut, 3/4 min. throttle and open her up wide open in the heavy stuff.

If the cutter does not have a "cintered" type vent fitting near the top of the box, money well spent to let her breath so she does not push a seal out of the front or bottom of the gear box.

Grease is your friend on cutters, plenty of it and hit the tractor spindles while your at it.

OEM equipped AC brush cutters had an over running "clicker" clutch, next to the gearbox, then a spring-loaded "rattle clutch", in front of that. State of the art, back then!

A-C cutter has slip type clutch that will let the blades wind down after the PTO is disengaged and PTO on tractor is allowed to stop, mounted in front of gear box and they were a great idea, just need to keep them adjusted.

Picked up a 15II that had an over-running clutch on it, been stuck on there for years as never serviced/lubed and it took some doing to get off the tractor.

Good idea to remove them occasionally to make sure the pin is not distorted and they need greased also.

Word of caution on the aftermarket O/R clutches; measure the length of engagement between the cutter shaft halves before cutting then down to compensate for the length of the O/R clutch as you still need a minimum of engagement on the shaft halves and they may have already been shortened.

On the other hand, not allowing the shafts enough room to slide together if you travel through a "dip" or ditch on a good angle can put undue stress on tractor internals for a heafty tear down for repair.