I think I finally decided to buy the stainless 20" Bushy but I would like to replace the front site with one of these

How hard is this to do?

If you are ordering from Bushmaster have them put it on when you order. If not...
-Drive out taper pins (left to right)
-Pull off gas block
-Remove rollpin and gas tube
-Install gas tube and rollpin
-Install new gas block/tube
-Square top of gas block with top of receiver (use machinist protractor or level)
-Snug set screws---If you want to set up/use irons
-Install irons/Zero rear sight
-Test fire and tilt gas block to get correct zero (windage) with rear sight
-Final torque/loctite set screws at this time
-If not using/fitting irons final torque at step 7
-You can install a sling swivel mt. in one of the set screw holes if you wish

PS---Killer gas block...!

Thanks for the info Noname!

You're welcome...!

Let us know how it turns out; I've been thinking of doing the same thing, myself.