The combination gas/sight blocks work well but obviously you need to cut down the barrel and turn the barrel down so that the gas block will fit where you want it and then you need to drill a new gas port.
The next thing you need to decide is if you are leaving the barrel in the trunion or not. Personally, I would want the barrel pulled even if you paid someone a couple bucks to do it for you. With the barrel out locating the front trunion holes is easy with a handy little tool that I will be making and selling here soon... The design is simple but you will see.
So, with the tool I much prefer flats with no trunion rivet holes. I have had too many flats that seem to be off otherwise.
As for headspacing, you want to check things before you screw it all together... if it is off, you need to fix it before it is all bolted together....