Before anyone can make a meaningful recommendation, more information will be required about the conditions. What fluid(s) is the metal exposed to at these temperatures?
253MA would have been my thought initially but obviously not in this case. I have used it sucessfully for burner parts firing into radiant refractory chambers at 1200degC but there was combustion (cooling) air flowing past it. When placed in comparitively stagnant conditions may be your problem.
Info I have suggests Sandvik's Alloy 601 (UNS No. NO6601) is good to 1200degC. It is welded with filler metal NiCrFe-3 or Sanicro 72. I think it may be worth contacting Sandvik and stating the service parimeters and furnace gas constituents.
Possibly 446 (UNS No. S44600)may work as it has a stated higher temp, rating than 253MA, but again the furnace atmosphere may have a bearing.
Good luck. Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au