The answer - I wouldn't use anything other than solder for these - is, as ever, a clean joint with the minimum of solder (I cut small bits of and apply the round the joint - tack the parts in place first). Low melt spreads a bit like warm butter, so you can, with a bit more flux, spread it around to some extent.
I see what you mean about adding the bolts which secure the crown plate. There is a single bolt head to the inner edge of each axleguard - not sure of the purpose of that detail, can you help?
I have to admit to gluing the sides together. Low temp solder never seems to flow into joints for me, I end up with a fillet of solder so I don't like low temp solder for joints with both sides exposed. If it was a van I would have soldered it.
I think they are the through bolts next to the transverse beams so probably not in the right places for the 9' underframe. I will check the photos to confirm if I need to move them as well.
I only know some stuff about some wagons in some time periods. If you had asked about Highland Railway wagons before 1900 I could have helped (and there is a thread). I agree with Adam’s answer but I don’t have any real knowledge about the late 50s or 60s tank wagons. I try to limit my BR wagons to pre 1957. When exhibiting I quite often run an anomaly or two to see if anybody notices, it can start conversations with people who do know what they are talking about.
