Why not? You're gonna be down there. If you have the money why not man. Me personally, I'd have a shop install the U joints though. The spider bearings can fall out REAL easily and can also fall out and compress if you hammer it in wrong.
I love mine. Part of the reason i got a built t-3650 instead of the t-6. Already had it, and wouldn't want to down grade to a normal one.
Most noticeably it makes it rev quicker, not necessarily huge amounts of gain in power. I didnt' feel power difference before and after since i have a supercharger (whats 10 hp on 450). But it revs a lot faster.
FRPP aluminum driveshafts come with u-joint's so you don't have to worry about them, & yes you just switch yoke's for the auto (take the wieght off the yoke while your in there, it come loose & cause problem's eventually anyway). as far as any gain's, on a relatively stock auto, you wouln't see any.
i have a auto with some bolt ons and gears im gonna get a aluminum d/s just for better basic performance of the car im not going to the track but i plan on doing heads and cam to get over that 300 rwhp mark i'll be happy with that its more then stock