I have a '61 C-20 with an H052 rear and have researched this a few times looking for an easy way to friendlier gears. I believe you are correct that the trailing arms are different width. I can tell you from searching fruitlessly for a bolt on trailer hitch, the 60-62 has a unique frame width. Also the axles are an unusual width, and I suspect you'll have to accommodate something like 1.5" less on the '61. I did hit a site via google search that had every axle width on a spreadsheet but didn't save it.