That sounds like you have the right way to go with your project. I put a Tasco Rubber Armored 3x9 (Delta style) on my HBAR flat top upper and I got a "Ghost image" of the front sight when I looked through the scope (very annoying). I ended up buying an A.R.M.S. mount for the carry handle and set up my scope that way. The lower receiver I have is an old SP-1, CAR-15, with the collapsible stock, so I also had to add a cheek piece to get my head up high enough to use the scope. It works pretty good now, but I would prefer to mount the scope lower on the gun.
As for my SP-1 Upper with the .625 diameter barrel, I have a Rock River Arms flat top receiver coming and my idea is to put the .625 barrel on it with some sort of gas block that does not have the front sight. I plan on adding an EOTech sight to this upper and I did not want the front sight getting in the way. If all else fails on the gas block issue, I will end up getting an M-4 barrel for this project as they are .750 under the front sight and then the YHM railed gas block will fit.