Well...There's a whole lot of "personal opinion" required to answer your question. So here's mine.

Don't under estimate the strength of the a833 in the cast iron case. Properly prepped, and most will say 600 ft/lbs. If it's going in the Scamp then no floor modifications should be necessary. Liberty Gears can help with faceplating or pro-shift modifications for quick shifting on either transmission. It will still require a clutch to shift.
http://www.libertysgears.com/

Passon performance has some really nice parts to beef it up.

The key to keeping any transmission together lies with the clutch first and formost.

Both transmissions have a major drawback in that they are helical cut gears; less noisy. For real strength and reliability a spur cut gear set is required. G-force, Jerico, and Liberty all make transmissions with spur cut gears.

Now if only someone made a spur cut gear set for the a833 case all these aftermarket 4-speeds would be a second choice.