How do I mount to engine stand?

Pictures of the thing stuck to the flex plate might help.

Usually a clutch is bolted to a flywheel not a flexplate, is your dart a manual transmission or an automatic?

If your car is an automatic and the "something else" looks like a giant donut then it's a torque converter and is held on with 4 large bolts (3/4"-13/16" from memory) through the flexplate.

You will need to rotate the crankshaft 90 degrees to access and remove each one from underneath the engine.

Sometimes these bolts are sixteen sided but a regular socket will work fine on them.

Attach the engine to the engine stand with high tensile bolts into the bellhousing pattern. Try to get some the correct length.

If the holes in the engine stand are too large for the head of the bolts, use THICK washers.

When bolting the block to the stand, try to get the centre of gravity of the engine close to the pivot of rotation. If possible adjust this again after the cylinder heads are installed.

This just makes it easier to rotate the engine when assembling it. (without it wanting to go upside down every time you unfasten the stay)