The bolt is an M8 x 20mm
That means it is not a crank bolt.
Could be a coupler to flywheel
May be an inspection cover bolt
May be an accessory drive bolt
May be an accessory bracket bolt
Spend $11 on a borescope for your phone and check the coupler
Yes, Gen 1/2 SBCs and the 4.3 until the metric block had used the 7/16-20 flywheel bolts
The Metric block used M11x1.5 crank bolts like the Gen 3/4/5 SBC
The 9.8 metric stamp on the head points to the 4.3 90M metric block (1999 and later)
1991, the block was changed, 1- piece RMS crank, moved all the tooling to Mexico, consolidated designs of rotating assembly with other GM motors
The flywheel went from a 153 tooth to 168 tooth
So get a complete bobtail
What size is the bolt? M6? M8, M10? How long ( under the threads) 15mm? 20mm?
That will help point you in a direction.
Other than that, as posted above, it's one of 50 or so metric fasteners from the motor.
