[RockHQ]

1. Did you disconnect your vacuum advance when checking your timing at idle? Do it (and remember to plug the vac. port you're using) and recheck. Set to 8-10° BTDC for a baseline then fine tune from there. Just because you have the rotor pointing at the same direction than with the old distributor does not mean your distributor cap poles are aligned the same way, or that you have the dizzy housing oriented the same way.
2. Double check your plug wires (firing order!)
3. If you converted from a points ditributor you can not run power to your HEI from the old coil positive (15) wire, because there's a resistor to limit voltage (prevents excessive points contact wear), you need to have full 12 volts to your HEI distributor. You can do a quick check by running a jumper wire from your battery positive to the HEI bat-connector, then see if that helps. You won't be able to shut off the truck from your ignition switch so you'll need to pull the jumper wire.