Is your engine "fer sure" all stock? By that, I mean, do you have a mix of late and early parts?
(Early SB had the water outlet on drivers side, timing marks on passenger side. Later switched sides. This means that if you have an early balancer/ late timing cover, OR late balancer, / early timing cover, you timing marks will not be correct.
ALSO balancers can SLIP the outer/ inner section so that the marks are wrong. It might be a good idea to check your timing marks, EASY to do
Either build or buy a piston stop, like this:
http://www.jerrybramlett.net/images/pic_installation.jpg
http://image.mustang50magazine.com/...+degree_four_valve_motor_cams+piston_stop.jpg
Remove the no1 plug, and make sure the piston is "down a ways". Look, flashlight, or check with bic pen, etc
Remove the battery ground for safety, and screw in the stop device.
Wrench the engine around until it stops against the device. You may have to play with length the first time, and you do NOT want the piston "at the top," but rather, "down a ways", so "longer is better."
When you reach this point, make a temporary mark onto the balancer directly under TDC on the timing tab
Now rotate the engine backwards, until it stops once again, and make a second mark
Your true TDC will be halfway between your temporary marks, and if the original is accurate, that is where it will be.
I found this video if you can stand the Ferd BS, and remember, what he's doing here, no1 on a Ferd is on the "wrong side"
Timing - Part 3 - How To - Using a piston stop to find true #1 TDC.wmv - YouTube