rollerize 904 help

Is the trans currently installed and driveable?Then it is a simple as measuring the 1-2 shift rpm drop after TC lock up is achieved.
The regular 904 has an rpm drop to 59%, while the low-gear units have a drop to 56%. So drive to 4000 rpm under power in first gear and make note of the exact rpm. Then stuff it into second , slow down to maybe 10 mph under the noted mph, and accelerate briskly back up thru that rpm, noting the exact rpm. Then take the second rpm divide by 4000 and that will equal the rpm split.
For instance ; the 59% will drop to 2350, while the 56% will drop to 2240.
As you can see; accuracy in the method is paramount, cuz there's only 110 rpm difference. If your does not lock up until some higher than 2350, then you may have to choose a higher shift rpm.
Another method involves even more math, and is probably a little less accurate.

If the trans is on the bench
the easiest is to grab the transmission number off the panrail, and do a look-up. Also a lot of them have a great big ol' identifier cast on top of the bellhousing; I've seen A904, and A998. I can't say all have the identifier, cuz IDK.
All 904s were 2.45-1.45-1.00
All A998s are 2.74-1.54-1.00........I think all
All A999s are as 998s except they are lock-ups...............I have been told.
The few that I have come across all have followed this pattern.
Each has it's preferred useage.

If the trans is torn down then it's obvious by looking at the sun-gear. The 904 sungear is the same from top to bottom, while the low-gear sun has two different ratios , and looks stepped.