Make a piston stop out of a old spark plug by knocking out the center electrode and porcelain insulator and use a long enough 1/4 bolt to hit the piston top around 20 to 30 degrees before and after TDC to make sure the dampener has not spun the outer ring with the TDC timing line on it
I had a old 318 truck poly motor car hauler backfire in the driveway when cold making the timing change jump one tooth, it was real lazy and I couldn't make it run properly until I replace that timing set
Good luck
Let us know what you find out about what went wrong