Doesn't numerically higher mean lower gears.
The 555/568 have the highest gears (of the ones that matter) followed by the 3.77 523, then the 3.50 523. Generally the low FD trannies have higher gears in order to even things out a little.
The 3.77 523 gears are stronger and more evenly spaced than the 3.50 gears so that is also a concern. Of course you won't be using the crazy 5th gear that is really widely spaced from 4th gear on the 3.50 gears so you probably don't care.
Remember you have to change the speed gears and the input shaft because the input shaft is a solid piece. Or conversely you can just change the ring and the main depending on which transmission you ultimately plan on running. This means you end up with the small input shaft in order to run the 3.50 gearset so you will probably also be looking for a different clutch...
If you do swap to the 3.50 gears someone else might want the 3.77 gears and input shaft because they are stronger and also a nice option for highway driving.
If I had a set of 3.50 gears laying around I would trade you and build a 3.77 geared 3.50FD tranny which is a nice hybrid IMO because it isn't geared so low and also is the strongest 523 combo.. I would have done one but I already have a 3.77 geared 568 with an OBX (slightly lower gearing than the 3.77gears on a 3.50FD).
-Rich