Couple ?'s for tranny guru's on 1989 A555 tranny

[4 l-bodies]

I am in the process of gathering parts for a 1989 520/555 hybrid (350:1 final drive) buildup and could use some advice. Does the 1989 A555 tranny need the chrome-moly end plate for a HP hybrid buildup? Does the 1989 not share the same mainshaft as the later 523/568?
I keep reading that the 523/568 don't need the endplate, but the earlier 520/525/555's could use them. So what changed besides the shafts, bearings, and synchros that mandates the moly end plate use?

The other question is should I be looking at the later large spline 523/568 hybrid for a HP build? This would be on 11 second and faster vehicles. I need a 350:1 final drive. A 377:1 or 385:1 is not going to work, as traps speeds are anticipated to be pretty high. I know this question is somewhat debatable, but would like to hear other peoples opinions on which would be best to build. I personally like the feel and shift quality of the A555 during full throttle power shifts gear changes vs. the A568's.
Thanks,
Todd

[moparzrule]

Some 523 trannies came with 3:50 gears, pretty rare though.

And yes the 555 needs the endplate, especially with being a hybrid because the higher gearing puts more load on it.