This is a frame jig that I recently built for another framebuilder. The materials cost for this was about $650. The manual machining time was significantly more than that, but your labor is free if you're building it for yourself.



The biggest problem I see with your design is the extrusions mounted to the spine. You need some way to key them to the spine, so they don't tilt out of alignment when you loosen the bolts to slide the extrusion. As soon as you loosen those bolts, that extrusion is going to tilt and it will be a pain to square it up and hold it there while trying to tighten the bolts. You could fasten a square plate on either side of the spine to keep it square with the spine.

How thick is the plate for the spine? is that 3/4" aluminum? I would go up to 1" for something that is responsible for keeping those long extrusions aligned.

Your tube supports should work fine and the single set screw in the bottom bracket cones should work fine also.