I need to get a flange gasket cut to fit between exhaust sidepipe and exhaust manifold. What temperature gasket material should I request - it will be about 18" from the head. The suppliers have suggested 450 deg C material but I just want to check. My gut feel says closer to 650 deg but that will increase the cost.

At 18" from the head 450 degrees will be fine. Mine are around 12" and I used ordinary household silicon sealer as a seal. 1000 miles on and it's still sealing