I believe
@perfacar raced 273's and I think he would know. I'm not sure you would need the 1.60 exhaust valve. I'm just getting back into research, but I am looking at 5/16 or 8mm valves for cheap, light, and quality alternatives for valves. The +.100 Magnum valves look interesting. I always used Chrysler or aftermarket stainless steel valves with 3/8 stems, but that was a long time ago.