i got it now. [i think ] if the correct coupler can't be found, and you need to machine your shaft, instead of welding on it, you should consider a 3/4 "double d" on the end you want to weld. very common from borgeson, and with whatever spline you need on the other end. or double d on both ends. basically, two big flats on a 3/4 [or 1"] shaft. when i size shafts for that kind of joint, i make the flats .001 smaller than the flat's female size.
i'm pretty sure other joint manufacturers have this double d as well as borgeson.