ntsqd; ok, you win
my (hopefully soon to start) project will have straight 10.125 x 3.062 x .312" frame beams (257 x 77.8 x 8.0 mm for the metric guys)
it is a MDT so the frame is "only" HSS and not heat treated (as is basically standard with HDTrucks)
this means i have to spread the load over a larger area (more bolts) backed by a rather large flanged cross-brace to avoid any kind of frame webbing flex.
so my bros advice is clearly towards a 3-point (with the rear trunnion at mid frame level and as close to the rear axle as possible).
right now we are looking into a idea to may use two rubber cushioned trunnions (one integrated into the suspension mount cross brace and one towards the end of frame)
to may spread the punctual load in the rear over a larger area.
i will post a sketch later on ...
Keep in mind that once more weight is on the truck the suspension will do more than it does in that pic and less frame flex. I cant tell but to me it looks like the suspension isnt giving at all?