I'm planning on using the high temp epoxy as a filler around the biggest pieces of aluminum that I can reasonably fit into the spaces, so the epoxy becomes just a bonding agent and gap filler. Being in Canada I am encountering again the conundrum that what is readily available for shipping online and/or in store just 150km away on the other side of the border is not available to me here. J-B Weld offer a "High Temp" 2 part (i.e. liquid) epoxy but I can't seem to buy it in Canada. J-B Weld do offer a High temp epoxy locally, but it is a "hand knead" type and it will be too "stiff" to fill in the tiny spaces I want to flow it to.
Permatex in Canada advertise a high temp (500 deg F) epoxy that I'll buy and try on a sample and subject to a few oven cycles (don't tell my wife!) before filling in the cavity in the head. Another reason for me to venture in to the Bermuda triangle of tool stores up here - Busy Bee, Princess Auto and Ottawa Fastener Supply (withing 1km of each other). If I don't re-appear its cause I'm stuck in a loop roaming the 3 stores...