[WFBowen]

Scutty:
Double check your fuel system, especially the pump for delivery. Can you determine how much fuel is in the bowls? During the rebuild did you do any modifications to the motor (cam, etc.?) Are you absolutely sure your valves were adjusted correctly? Pull the rocker covers and visually confirm that, on tdc firing stroke for that cyl, both valves are closed. Double check all vacuum ports for leaks. I'm assuming you no longer run the smog stuff on your 68 (throttle positioner, air pump, etc.)
Along with what Bret stated, some members posted here a while back concerning the wrong thickness float needle assy. gaskets in recent rebuild kits which threw everything off. After shutting off the motor, you can carefully stick a dry toothpick into the sight holes to see where (&if) the fuel level actually is.
Keep us posted,
Bill.