Easy (?) OD Auto Install

I have a A518 needing rebuild waiting in the wings for my B-Body. Currently running 904. The idea of a lower first gear and the lighter trans so the 999 has me thinking. Hope I am not hijacking but can anyone comment on the strength of the A999 and what donor vehicle to look for? Assuming they are hydraulic and not electronically controlled.

Edit.....strictly a street car with say 350HP 400TQ ish