Piston Stop to bolt-on to the block?

If you have any decent scrap laying around you can make one. Just weld a 5/16 nut w/ a bolt threaded through it in the middle of the span on a 1x.250 strap, drill two more holes to pass the bolts through into the head bolt bosses, and you got one.

You can do what Mike said, or if you don't weld, you can determine TDC with a dial indicator. I like the dial indicator method better anyway, it reduces the amount of specialty tools I have laying around (like a piston stop!).

Decide on a measurement- let's say .100". With your dial indicator and a degree wheel, record crank degrees when the piston is .100" in the hole BTDC. Then turn the engine up and past TDC, until the piston again drops .100" in the hole ATDC. Record this crank angle. True TDC is the middle point between the two crank angle measurements. Easy.