Engine Combo's that turned out terrible?

My 273 bracket racer (stock heads) ran a best of 12.72 @ 108 MPH. Notched the block for valve clearance, and opened up the valve reliefs in the forged pistons in order to install 340 cylinder heads with 2.02 intake valves. Everything else remained the same, including the 340 Tarantula intake manifold installed on the 273. Never broke out of the 13 second bracket after the switch.