Assembly is going to be extremely tough unless one of the grooves has an open side that you close up after assembly. Otherwise, the o-ring has to be compressed way beyond its design intent to slide down to the second groove. The chances of the seal being damaged during this process are high.
This is a tough question to answer without knowing the specifics of what design and materials you are using today. There are good FKM elastomers that will take alot of abuse at up to 200°C, but its hard to make any material or design recommendations beyond that.
