Late timing (which aggravates) rich mixture results in high exhaust temperature. That knocks the valve seats out of round and can warp valves. I suggest timing never any later than 32 degrees, nor more than 36 degrees. I don't know what heads your are running so i can't tell if you have big seats or not. Many chevy heads had little seats and if that is the case, you'll chase valve jobs all the time. Also you need to check your valve spring seat pressures are at least 220 lbs on the seat. More is preferred.
I have seen exhaust temps of 2500 degrees with late timing and rich mixture. No valve likes that.
Measure all your valves overall length to see if any are tuliping or stretching. Occasionally I catch an exhaust valve stretching .060 or more. But usually that's from a little lean in that hole.