I think the problem is a flathead engine can’t really respond to the improvement. Did you port the block to maintain the cross section between the valves and cylinder for gas flow? Not sure how much wall thickness there is in there also before you break thru.
No I was about 12 and just play-doh’ed the valve to head clearance and milled it with wet sand-paper on top of a saw bed (I assumed it to be flat.)
I didn’t know about or understand the differences in metal expansion rates or valve float. I might even have spun it over 3600 rpm...