A999 tranny same as 904?

Hey guys. I just found an non lock up A999 for sale and I'm digging for info about the OD benefits. I am currently running a 904 behind a warmed over 360 Magnum with 3.55 gears in a Bbody. I also have a 46RH/A518 than needs rebuild but I don't like the extra weight and fitment mods. What can I expect for RPM drop in OD? Thanks.

Edit: I should also add the small choice of converters for the 46RH. I believe I can use an y 904 converter with the nlu A999 as long as the weighting is correct. The A999 I am looking at is an early unit that has the higher gear set.