Overdrive/underdrive gears FCX24, limited choices due to portal housing?

[Whats_a_dickfer]

Throwing this question out there hoping maybe some of you with more experience or expertise can chime in. So with the fms fcx24/fcx18 platform, the stock portal gears are 14t/21t. The only alternate gears out there are a 15t/20t set, pretty minimal OD. Having installed them it looks like the internal dimensions of the portal housings are the limiting factor. Could this be the reason why nobody has come out with more options?

As far as the ring and pinion go, I'm no expert, but I just don't see anything inherent to the design of the axle that would prevent different gear sets from working. These are fairly popular little trucks, and it seems like everyone who owns them would be interested in better od/ud options. Thoughts/opinions?