OSN would often take my carefully drawn illustrations and re-draw them with a computer. No guarantees.
Trial and error is really the only way, but I do have a hunk of 2 1/8" dia. exhaust pipe cut to the proper SP angle. I can slip a 2" brass tube inside, and cut that way for a more or less good fit. The SP angle is different from the Seaboard angle.
What I now do is fly- cut two bulkheads to slip over the tube, solder the oil (coal) tank wrapper to them, slide the entire assembly on to the tube, solder, and then fill in from underneath. If you are given to plastic, that can be easy, and you can fill in gaps with putty. Even easier with cardboard. You never see that area, anyway. I fill in with metal, after making cardboard templates, and solder.
Lots of advantages to this method - squareness is much, much easier.
