The first thing to check is the bell mouthing of the guide. Ideally, set up a dial gauge and measure how far the exhaust valve head wobbles side to side, then divide by 2 to allow for the leverage of the length of the valve stem. Compare with permitted tolerances.
If the guide is within tolerance, then pay someone $20 to dress all the exhaust valves. That will set the mating angle to exactly 45 + 1/2 degrees. You then need a 45 degree carbide cutter to do a very quick clean up of the valve seat. The first few miles will then see the valve and seat mate very precisely.
If the guide is not within tolerance, then even with an adjustable pilot firmly anchored, you will never get a stone or carbide cutter to stay concentric, and the valve will never seal properly.
There is little scope for valve grinding paste these days, except possibly to confirm that a valve head really does match its seat.