Because of the way the crankshaft is made, the #1 and #6 pistons reach TDC at the same time. Ditto for #2 and #5, and #3 and #4. There is no way to determine which cylinder is at TDC on the compression stroke without looking at the valve action. All engines with an even number of cylinders except a few oddball V-6's share this situation. Two pistons will reach the top of their respective strokes AT THE SAME TIME. The only way to pick out which one is on the compression stroke and ready to fire the spark plug is to understand how the camshaft and the valves operate.
The triangle on the flywheel indicates that both #1 and #6 pistons are at the top of the stroke. I haven't looked at the crankshaft/flywheel mounting procedure in awhile, so I can't be sure if the flywheel could be installed incorrectly. If so, then one of the other cylinder pairs would align themselves with the timing marks. I think that is VERY unlikely, as all automotive engineers try to make their engines as idiot-proof as possible. If your timing marks at the flywheel are not aligning with TDC according to the valve action, it's very likely that the camshaft is installed incorrectly.
Jerry
