crank identification help

[69projectKAOS]

im trying to order a flywheel that will mate my lt1 t56 to my pre-85 sbc ( 2 piece seal) but now the question is , how do i tell if my crank is a 400 or regular 350 crank. my dad says that the guy who built the engine about 2 years ago said it was a 383, but from what i can remeber he said it was a 355. i checked the receipt from the motor build and called the builder but the said it was too long ago and cant remember. i need to order this flywheel in order to get my transmission in and dont want to buy the wrong one. are there any things that will tell me the difference between a 400 and standard 350 crank without opening the motor up?