The PTO for a flexplate has a counterbore to register on the crankshaft.
The PTO for a flywheel has a boss to register inside the flywheel itself. Flywheels are thicker at the flange than flexplates and totally cover the crankshaft register.
The crankshaft 3.58" bolt circle is the same for two-piece rear main seal (pre '86) SBC and BBC. One-piece rear main seal SBC use a 3.00" bolt circle.
The flange yoke does not have a square bolt pattern. If you rotate the drive line 90-degrees, will it bolt up? Could be, however unlikely, that you have different series drive line and PTO, i.e. 1310/1350 or some other old, oddball parts.
