First, you're going to need a couple of special tools. You will need a degree wheel that can be mounted to the harmonic damper and then fabricate a pointer out of a coat hanger or welding rod. Next, you will need a positive stop to screw into the spark plug hole of the No. 1 cylinder. You can pick these parts up from any of the mail-order speed shops. Summit Racing offers these tools; the 11-inch degree wheel, and a set of three bendable bolt-on pointers. Proform offers a TDC locator (positive stop) under PN 778-66792. With these tools you can accurately find TDC of any engine.
To prep the engine, you'll want to remove all spark plugs, the driver-side valve cover, and all the belts and pulleys from the harmonic damper. Install the degree wheel to the face of the damper. With the valve cover removed, rotate the engine slowly and watch the rocker arms on No. 1 cylinder. What you'll want to do is rotate the engine clockwise from the front of the engine. First, you will see that the exhaust rocker arm will open the exhaust valve and, as the exhaust rocker is closing, the intake will begin to open. As the exhaust is closing, and the intake starts to open—stop turning the engine when the rocker arms are equal. This is where the intake and the exhaust valves are open the same amount, or what is called overlap. This will be within a couple of degrees of TDC. Once you have found this location, fabricate a timing pointer that aligns with TDC on your degree wheel. Once you have done this, rotate the engine one full revolution, which will put the No. 1 cylinder at TDC on the compression/power stroke.
Once you have your degree wheel and pointer set up, you'll move onto using the positive stop. First, rotate the engine clockwise 100 degrees. Now you can install the positive stop into the spark plug hole without hitting the top of the piston. With the positive stop installed, very slowly rotate the engine counterclockwise until the piston comes in contact with the positive stop. Again, slowly—if you get after the crankshaft and turn it quickly, you can bend the stop in the spark plug hole and dent the piston. When you have found the stop and the crankshaft has positively stopped, record the degree location on the degree wheel. Depending on how far the positive stop is inserted into the cylinder, this could be anywhere from 20 to 50 degrees before top dead center (BTDC). Now turn the crankshaft clockwise one full revolution and slowly come up on TDC until you hit the positive stop again. Record the location on timing pointer and see where you have stopped. What you are going to do here is see the difference and adjust your fabricated timing pointer until you have equal degrees of rotation when you repeat back and forth from the positive stop. For instance, if the first timing mark you recorded was 20 degrees BTDC, and when you rotated the engine around to the other side it was 25 degrees BTDC, you would take these two numbers and add them together and divide them by 2. This would leave you with 22.5 degrees. While the piston is still against the stop, and at 25 degrees BTDC, bend the timing pointer until it lines up with 22.5 degrees on your degree wheel. Next, rotate the engine counter clockwise until you're coming up on 22.5 degrees BTDC and slowly rotate until the crankshaft stops. If it lines up squarely with 22.5 degrees, you have found TDC with your degree wheel and fabricated pointer. Once you have achieved the same degree readings on both sides of the positive stop rotation, you can remove the positive stop tool from the spark plug hole. Then rotate the engine until your fabricated pointer lines up with TDC on the degree wheel. This will be true TDC on your engine.
In your case, if you don't have a factory pointer on the front of the engine, pick up an adjustable Spectre timing pointer PN 865-4237, which will fit 6-, 7-, and 8-inch GM dampers. It is held in place by two of the front cover bolts. Adjust the timing pointer until it is aligned with the timing mark on the factory damper. If there is no timing mark in that location, then scribe and mark a new timing mark on the damper as you have found TDC on your engine combination.
We know this sounds like a difficult process, but it's really easy if you follow the step-by-step instructions. This is how you can find TDC on a fully assembled engine. You can find many examples online on how to find TDC and degreeing camshafts on short-blocks, which make the job much easier with a dial indicator. Hopefully, you'll be able to set up an accurate pointer, and this has given the masses the ability to check out their timing. Good luck, and be patient!