Forgot to mention you have 15" subs and a Blazer.
I have done street A in mine with pair of 10s in 2006 got up to 152.7 certified with a Z1 and 9910s. Not to mention i did that # from a wheelchair lol. Used 1 cube per sub common chamber and a pair of 6" aeros, subs firing towards the passenger side ports to the drivers side. This worked best in my blazer but have tried it in other peoples and port back seemed to be better in theirs. Truth is you will have to test that out yourself to know for sure. Cause in mine i gained 0.2 over rear firing port but other blazers seen it lose them 0.5! Also mine peaked 56 Hz, my ports peaked @ 57Hz on paper.
Now for you i would do like 2 cubes per sub common chamber to start. If you are going to use a slot do it external, you will see a gain in efficiency from that. Do as much port area as you can and tune in the 50-58 Hz area. I have never seen a blazer peak under 52 Hz for burps.
Here is a pic of my loudest street A box. Similar to what crx guys were using but doesnt work the same in a blazer thats for sure.