It's always a good idea to apply thin CA to all corners of the firewall before fuel proofing. Let it soak into any open cracks or seams. After that you can fuel proof with thinned epoxy. I like to put 3-4 cap head sheet metal screws on each side to pin the firewall. It's sort of a back up that I like to do. You can also pin it with hard wood dowels but the sheet metal screws take seconds to install. You bought a good quality kit but it's always recommeneded to go over the entire air frame with CA or whatever glue you like that penetrates. Sometimes they don't put enough glue on.