HAVE A 396 WITH BLOCK #3955272 DATE I-17-68 HEADS ARE 3904391 BOTH ARE DATE I-20-68 4 BOLT BLOCK 6223 CRANK . HOW DO YOU TELL IF 375HP OR 425HP EVERY 375HP I HAVE EVER SEEN HAD 840 HEADS MOTOR WAS IN 69 CAMARO ITS ALL FRESH WITH ROLLER CAM AND 42CC PISTONS HAD 750 CARB ON DYNO MADE 600HP.
3955272 is a 1969 396 block that was used in applications ranging from 265HP to 375HP. Since yours is a 4 bolt main, it was originally a 375HP configuration. (There were no 425HP 396 engines built in 1969.)
The 3904391 heads are 1967 only, and were used on the Chevelle and Camaro 396/375 engine and the Corvette 427/435 engine. You have a 1969 block and 1967 heads. However, since both your heads are dated I-20-68, it appears they may be service replacements. Can you post a picture of the head dates?