Get yourself a new head off Amazon or such. Cut about .090 for a new head to leave room for truing. A seasoned head you'll go more. After the head is seasoned, can play with valve angles and seat angles and improve dramatically upon the 28 head. There is more support near the pushrod area in the new design head that will be important if you are running a 33, as that area likes to blow out the head gasket. You can back cut both valves, do some port work, depending upon rpm change intake seat to 30° if lower rpm/stock cam. Get your valvetrain angles right. Index your spark plug, cut the ground electrode. All small things, but as superman said, enough of them add up.