Ken F.
I'll save Steve B. from wearing his fingers out on the deck leveling thing.
To do this job good and right...and make a deck tune up "hold" you will need to fix all of these areas.
1) Remove the QA hangers. Get some 1/8" x 3" flat stock, place it over the hanger locations (after you have beat them flat where the bolts have tried to pull out of the skin), and weld the whole strip (20 some inches) across the back of the deck with short weld seams...this will make the rear hangers rock solid and stop future abuse. re-drill and re-attach the QA hangers.
2) grind out and reapir any cracks at the right rear gauge wheel.....reinforce as necessary.
3) Fix any other flaws in the metal......bang out the front lip of the air tunnel if it is dented in, etc.
4) with the deck on it's back, install new blades (GATOR Blades are the prefered blade of choice for 44" and 50" decks IMHO) and start checking them tip-to-tip for level cutting, use a long straight edge to make sure all of the blades are in the same plane. If any of the spindles are tweaked, whack the deck skin around the spindle with a BFH as needed to correct.
5) Flip the deck back over and set the 2 outer blades on 2x4 blocks, on edge (along the long axis of the tractor). This should have all of the blades in a plane 3-1/2" off of the floor and parallel to it. Measure the distance from the gauge wheels to the floor (in the same holes)...it should be the same...if it's not, persuade them back into shape as necessary....BFH, etc. BUT NOT WITH THE DECK ON THE 2x4's...you'll bend the blades.
6) Install a front gauge wheel kit if you so desire. Weld the brackets on so that with the gauge rear gauge wheels are in the bottom holes and the front "ball wheels" are in the bottom holes they are the same distance off of the ground when the deck is positioned as in step 5
7) Mount deck, with tractor on level surface, tire air checked...etc. as I describe in the FAQ. Rough level fore/aft using the front eye bolts. You make run out of adjustment in the eyes before you get the front blade tips up far enough to be level.... in that case remove the subframe and bend the "pickle fork" looking tail on the front eye bolt carriage (the fork that slips over the tractor's lift rockshaft) up in the air about an inch...use a torch... and then repeat the process. level front blade tips to within 0" to -1/4" of the rear blade tips.
8) Level side to side using adjustable deck hangers (see sponsors or your local dealer). Level side - side to within 1/4". Recheck fore-aft and then tighten everything down.
9) Roughly set desired cutting height at blade tip. Set gauge wheels in holes so that they are 1/2"-3/4" off of the ground, fine tune deck final deck height to get the 1/2"-3/4" measurement "dialed in". Set and lock the depth stop cam in the tractor's lift.
10) Go mow your yard and marvel at the pool table like appearance.