39700010 was used from 68-mid seventies. Both two and four bolt. Piston no. is GM number for a cast flat top production piston.

So for sure you have a Chevy block with stock pistons. Most of these were around 275 hp. Unless the crank is steel. Then it could have been from 275 to over 300.

To tell if the crank is steel, the parting line that runs from the front to the rear of the crank will be about 3/8 ths inch wide.. If the line is just 1/16 wide or less, the crank is cast iron.

Factory hp. means nothing anyways if you are going to build this motor. It will make a good basis for just about anything up to 400 hp.