The bottom bracket thread is 1.37" x 24 t.p.i, as for the bore i am unsure, but if you are planning on making your own bb shell , it will be hard work due to having a left hand thread and the right hand thread, also theres a couple of ways of doing it, if i was to do it, i would bore the material to the correct size then i would screw cut the threads left hand and right hand......... the other option is to tap it, however, the taps that are readily available are for only cleaning threads, to get a tap that would cut material from scratch in that size would be like trying to find a needle in a haystack, however, like you said your best option is to get a frame and chop off the bb and headtube then just throw them in a lathe to clean off all the old weld and shite on the outer diameters regards ash