There were three different positions for timing tabs on the SBCs. From straight up on the crankshaft, the timing tab locations were 5*, 35*, and 43*. Sounds like the engine tab is at one location, while the damper/timing wheel is for a different engine. Most aftermarket dampers/timing wheels are at the 35* position. That sounds like what you have.
You can verify this by looking at the front of the engine pully (using an inspection mirror), and rotating the crank until the keyway for the front pulley is at 45 degrees (1:30 o'clock as viewed from the front). That is TDC on #1 cylinder. Where is the timing mark on the wheel?
Easiest fix is to replace the timing wheel with the correct one, or put a cover on the damper with the correct markings. Another fix is to apply a timing tape to the existing wheel. Another way is to set an advance timing light with 30* more advance than you want. This should rotate the timing mark you see to compensate for the timing wheel being 30 degrees off.
Here's timing tape. You just apply it 30 degrees off the existing marks:
http://www.jegs.com/p/Mr-Gasket/Mr-Gasket-Balancer-Timing-Tape/746079/10002/-1?itemPerPage=60&pno=1
