PoulanPRO PP175G42 Lawn Tractor

Hi gera - welcome to the MLF
Usually, when a mechanism is engineered with slots, it is presumed that the mid-position is nominal. The slots are provided for assembly variation and end user adjustment if needed,
I would position bracket 172, initially, in the mid-slot position and as vertical as possible and then check for proper linkage movement. The link should move / swing freely and not change angle drastically. It should not exert usually force at full extent either. If it is shifting, I would use close fitting washers (smallest inside diameter) and sufficent large diameter on both sides of the bracket and chassis and use a Ny-lock nut to retain the clamp load. Get a grade 5 or grade 8 bolt if you wish for additional torque capability. As a extra step, once you are happy with the positioning and linkage movement, you can drill a small hole between the two slots, and through bracket 172 and the chassis and the screw a hardened sheet metal screw (or drill and tap for threaded screw) through to pin it in place.
Just remember, you have to figure out what is applying enough force to move it out of place first and then fix that.
good luck, keep us posted (with pictures too!)