The 63A gearbox, as far as I know, was used only in left drive cars when new, so just be aware of that.

It was made only in the dogleg shift pattern I believe, so that should be good, but at the end of the day, it, like both versions of the 60 series 5 speed, is really just a four speed with integral overdrive in fifth, unlike the small & light 56A which is a true 5 speed.
The overdrive 5 speeds were not really intended as a performance option but as an economy option but fortunately, with lower differential gearing, some satisfactory use can be made of them.

The 63 series gearbox design was originally intended for use behind the L series engines with their high torque & heavier vehicle weights. Even behind an L20B, which is the real torquer of the bunch, they seem to have no trouble, so behind an A series, they are [in most cases] both bulletproof as well as being too big, bulky & heavy to really be justified.
Their larger & heavier gears suck more power from the engine to turn them over than smaller ones too, so while the 63A is a bloody good box, an exceptional box, is it really justified in most cases?