Larry,
You can purchase a 'nut tap', 3/8x16x6"OAL for about $15 + s/h.
What you then need to do is to reduce the diameter of the shank behind the flutes/threads to just less than the dia. of the tap-drill. In this case, anything less then .3125".
I don't know why you would need an 8" tap for wght. bolts. The longest I ever use is 5" and very rarely. If you want heavier than that, go to a larger dia.
