> 1) I see that there were two archbar truck sizes, 3'7" and 4'6". Understanding that the refrigerator cars used Bettendorf trucks, how were the different archbar trucks used?
This is a complicated question (and I was rather hoping someone else would chip in ) but here's my version of it. The 3'7" archbars appeared on 3000 series box cars, 5500 stock, high sided and drop bottom gons and some flats like the 61XX and so on. But it's the flats that have the greatest variety. The 4'6" appeared on latter TCX and Conoco tank cars. Just to add to the variety the 5900 series stock car (34') had 4'8" Andrews trucks (which were swapped onto UTLX cars for a time) while fishbelly flats had special 4'11" ones (both these cars are available as PCS kits). There was also a 4'8" archbar which appeared on some flats and gons.
Caboose trucks are different. They were 3'7" but in order to make the ride more comfortable for the crew they had transverse leaf springs instead of the four coil springs each side. The best information on all of this is in Bob Sloans 'A Century plus ten of DRGW narrow gauge freight cars'. It's in print.
If you're making these kits up here are another couple of things I did when building mine. First in order to make the grabs all protrude the same distance from the body I cut a thin piece of card narrower than the grab widths and used them as a depth gauge. The other thing I did was to make the holes for the inner truss rods about 1/8" each side closer to the queen posts. It doesn't show at all and helps the trucks clear the rods when swiveling, especially those with the outer brake gear fitted on outriggers like Grandt lines (you see these cut off sometimes). I
I think the tip of fitting couplers and trucks right at the start is an excellent one and will use it from now on.