Budget racers quickest stock piston and head motors

The 340 in my 68 dart has stock replacement pistons in it, stock X heads that looked to be opened up a little near the valve seat with some kind of machining but no porting. I did a rering and bearing, installed a small 235 @.50 solid cam with around .480 lift, 273 adjustable rockers, air gap intake, holley 750 dp, stock mopar electronic ignition, cheap flow tech headers, 3800 stall converter, 410 rear, ran best of 12.90's @ 105.