here ya go...I have:
1) You seem to already know it is 5/8ths round stock (that's what I got too)
2) 18 5/8" long
3) 1/4" diam holes 1 1/4" on center from each end and on the same plane
4) slight dimple on the rear end of the rod (maybe just wear from the ball bearing that sits in the rear yoke but I'm guessing it was on purpose since the ball is supposedly there to aid alignment).
curious as to where yours broke...seems to me that the 2 rubber "tire" discs should have failed before the shaft (in addition to serving like U joints, they are supposed to be like shear pins on an outboard motor). did it break at one of the holes due to rust?