I have the same rear sight on my 590AI, and am in the process of building a 930 which I plan on configuring the same way. I had my gunsmith install a short section of 1913 rail in front of the LPA rear, and he said he was only able to use the 2 existing factory screw holes since the receiver gets very thin in front of those holes. I guess it's thinner to accomodate the barrel that projects back there. In any case, I was only mounting a Burris FastFire so I didn't need much rail, and the 2 screws seem to be holding the rail in place since the FF doesn't weigh much. Will definitely be sending the barrel to Casey for front sight installation.
Where did you get the Trijicon front ? I was thinking I may just use the factory front from a 590AI.