I am for sure to an engine guru but if your setup ends up needing it I have used/use GM .016-.018 shim gaskets with aluminum heads and have had no issues with them. Make sure that everything is flat, use good fasteners and make sure of your torque values. It will depend on where you end up with deck, piston/head clearance and all the other things you will want to consider to maximize quench while avoiding "things" getting together. You can get MLS gaskets from Cometic and others that come in around .025-.027, I have used hem also and they are nice, I also think the fire ring is fine at the edge of the bore as long as there is not overhang and obviously as long as you get a good seal. I would like to see a picture once you have time to post one to see exactly what you're dealing with.