I only know this because I just figured it out.......... finally got my upper in after 3 months of waiting, started looking for sights and saw that there were 2 different heights
Not true. There are gas block height sights, here are a couple
http://www.midwestindustriesinc.com/index.cfm?fuseaction=category.display&category_id=79
http://www.gggaz.com/flip-up-front-sight-for-dovetailed-gas-blocks.html
There are front sights made specifically for use on the gas block.
I actually just bought these http://www.theoutdoorstrader.com/threads/350632-FS-FT-midwest-industries-OD-Green-ERS-BUIS?p=1884181#post1884181