There used to be a guy at DBSForums, Raphiel, who operated a private cable system of sorts in Venezuela that used a couple of 12 foot dishes pointed at two different DirecTV satellites in the 101 slot that were typically only 3/10 of a degree apart.
Last I knew, DirecTV would not allow commercial installers to use dishes larger than 4 feet in diameter because they were concerned that they might not properly target two or more of the satellites in the 101 slot, but if DISH is only using one satellite at 129, the "figure eight" drift of that satellite should not cause any problem with any size large fixed dish that you might use.
Feedhorn positioning is critical in large Ku set-ups, and what really makes the placement demanding is that the abandoned C-band dishes have lowed f/D ratios than do typical Ku dishes, so the signals reflected from the periphery of the dish come into the feedhorn at a wide angle. An optimal feedhorn assembly for receiving Ku from such a shallow dish is probably a feedhorn in which the probe is out in front of the LNB, rather than recessed back in a waveguide tube.
I was once called out to service the headend of Muslim Television of America. They have a gigantic (maybe 25 to 30 foot) polar mounted dish fitted with a Ku feedhorn that they are dissatisfied with and asked me about peaking it for them. I told them I wouldn't touch it with a ten foot pole. In fact I couldn't reach it with a ten foot pole. nor would I attempt to with a thirty foot gantry, since if I move it even a fraction of a inch, I might lose the signal completely and never be able to find it again.