I had always heard when I had an O-290D on a Wag Cuby that you could fly cut the case for the O-320 Cylinders and get the thicker valves to eliminate ine of the O-290 issues- gulping thin valves.
The other issue is some say the camshaft was an odd length, almost too long, and flung oil toward the breather- so we all had air-oil separators. Maybe the oil migrated up the accessory case gears and made it to the breather.
If the cam lobe contact area on the lifters is wider on an O-320, I would lean that way.
I had no issues with keeping the dry tappets adjusted, just check/set at annual, added maybe 30 minutes.
You'll need a bigger carb to go to O-320 Cylinders, from a 3 series to 4 series.
Mahlon!?