then get a replacment tab. they make bolt on ones. however all you need to know is where the big V is on the tab and use timing tape for the correct damper size.
correct way to find TDC is to use a piston stop and degree wheel, however on a stock SBC, I would probably pull the motor and replace the timing cover. your looking at about 4-6 hours worth of work to drop the drive, pull the motor, swap out the front cover, clean up the rust, repaint, re-install the motor and align the drive.