I thought 54/55 had rear cab shackle mounts like 49-53? The assembly manual pictures you reference with different mounts are they for a panel truck or suburban?
The sheets (1.00 and 2.00) don't differentiate between pickup/panel/suburban, so it's possible the outboard body brackets shown are only for panel/suburban, but there are no notes to indicate that, although sheet 8.01 only indicates 3105, 3106, 3107, 3112 and 3116 models (all long body types), and not 3104 (pickup). I haven't been under either a panel or a 54 to look at the frame.
So it's likely you're right about the 54-55 having rear cab shackles.
Sorry for the excursion into left field on the body mounts.
