how tough really is the 63A transmission? 100, 200, 300 ,400hp?

Transmissions are not rated for HP. The 63-series, it is said to be good for 175 lb/ft of torque. Nissan put them behind L20B 2-liters.
Gearbox strength

nissan may have put them behind stock L20bs(thought they were only behind L16s though), but a worked L20b would blow it to pieces

Depends on how "worked" it is. If it's all high-RPM work, the HP will be high, but torque may not exceed 175 lb/ft. On the other hand, if it is medium RPM "worked", the HP will be higher, and the torque will exceed the limit.

L20B trucks had the 71-series transmissions fitted. So yes ...

pretty sure all L20bs had 71 series transmissions, which are a fair bit beefier than the 63 series

but they should stand up to any NA A series and most boosted ones you would think

I run one behind my supercharged A15. I do my best to break it, but to no avail. I'm a bit hard on synchro's though. If the box was otherwise in good order, it would take quite a bit of punishment to break one I should think.