Is it worth the effort to swap from a 904 to a 999. I saw a a999 on craigslist for 150 and needs rebuilt and comes with converter. Pics look good though.
I believe the a999 uses an electronic lock-up torque converter. If so, it is not possible to create a torque converter with the small knob to fit the small pilot hole in the '63 slant six crank.
Been there; done that.
You'll need to change the crank out & replace it with a '68 or later forged crank.
Also, the tail shaft housing is larger in diameter, requiring ether cutting the trans tunnel or modifying the cross-member to lower the back of the transmission, which might require playing with pinion angles.
To keep the push button control for the later linkage transmission, you'll need to send your push button gear selector assembly to Imperial Services (http://www.imperialservices.net/) and purchase their conversion kit.
If you want this badly enough, it's worth the effort.