Staw: My dad has an old Trueline Jr. loading press. All of his dies for the 8 or so cartridges that he loads, also fit the 310 tool. So if you find a set of the trueline dies, they will fit the 310 tool.
An old gun smith I knew would make straight line loaders for odd cartridges. They worked a bit like the lee loaders, but looked like the ones made by Harry Pope.
About ten years ago, I contacted Wilson Tools to see about a limited run of 45-70 and 50-70 straight line loading tools. The person I talked to quoted me a price of almost $125 a unit to make them in lots of 25. And they sell most of theirs for around $30.00 wholesale. I figured they just didn't want to be bothered.