the reason tha Mark had to modify was that the x member is a little different between a long and a short bed so he had to modify it a bit. you may find a stock one but they usually want a lot of money for them.
by looking at what mark has there you could fab one up. have someone weld it up for you.
I have already told you that 63 frame was different so that isn't going to do you any good
this may help you .
http://www.persh.org/pickup/700R4.htm
ron