So the V150 model is finished and in my Shapeways store.
Here's how it looks with a coat of paint and a few bits of additional dressing:
More notes on scale; these are old Morrow Project miniatures from the 90's, thus the Ral Partha Dwarf proportions. Technically 28mm, and as you can see, they seem roughly proportional with a vehicle in 1/56 scale. At least, it is as close as I could get to 1/56 by working with the quoted length of the hull, from the blueprints I had available.
To recap the scale process: I scanned blueprint images and cropped and scaled them to be square and dimensional to each other. I took the pixel length of the largest scaled item that appeared in any one drawing and extrapolated the real-world dimensions of the blueprint space.
Within Carrara, I set the working box to the size of the blueprint space; this meant that if the model I was building was lined up accurately on the vehicle in the drawing, it would be the correct real-world size. This worked out, to within a small degree of error (a fraction of one percent error).
The two biggest problems I had within Carrara were, first, that I was working metric while most of the dimensional information was in feet and inches. So a lot of multiplying by 2.54 to get the right units into the modeler. The other is that Carrara, stupidly, only displays two digits to the right of the decimal point. This means that a vehicle sitting within a ten-meter working box can not have any numerical measurement that is smaller than 10 centimeters.
Which is ridiculous! Any of the detailed parts, then, could only be lined up by eye against a grid (which could be set finer than 10 cm). Once again, it is really stupid software for anyone doing a model more elaborate than the Linux penguin.
The drawback of the method is that when I moved into checking for printability I had to divide by 56 all the time to find out what the print size of various parts was going to be. Finally I just reset the grid to be at 1 mm in the final print size of the model (aka 56 mm in world scale), and eyeballed everything to make sure I was staying within the Design Rules.
Since I knew the longest dimension of the completed model in real-world scale, all I had to do is divide by 56 to figure out what the size of the scaled mesh should be. The actual export from Carrara was at arbitrary scale (Carrara doesn't do scaled obj format). But all I had to do is type the correct longest dimension in the scale box in Hexagon 2.5, and the stl exported from there was correctly scaled for the Shapeways printers.
The last scale trick was to line up all critical-fit parts the same way they would be when assembled (as the printers aren't always the same degree of accuracy in x, y, and z axis), and export them together (to make sure they are all scaled the same ratio and will fit properly after printing). In this case, I attached the different parts together with sprue to make it easier for the lads and lasses at Shapeways to handle what otherwise might be small, fragile parts.