a833 into a '63 Valiant

You can do the 4 speed with that slant but you need a dedicated 4 speed slant bell, there are 2 cast numbers, don't have them in front of me, I have one. They are not cheap nor easy to get. I bet your 3 spd has the provision for a floor shift??? IF so that is the most economical route unless you just want the 4 speed. There are also A833 OD trans and they take a dedicated bell, many were found in F bodies ( Volaries, Aspens,) some pickups.
LIke said, Brewer Perf. is a great source of info and parts.