Help converting my 64 to 66 23 Spline 8A33 to an overdrive ?

Hello guys.

Thank you for all the help so far

I have a 64 to 66 A833 that i would like to convert to an overdrive style. Ive read that Passons makes a kit. And brewers also make ones. Which would would be the best that would not require me to modify the tailshaft ?