428s are great engines, and a complete, rebuildable one is easily worth the $600. If it's an SJ and that's the original engine in it, chances are it's a 4-bolt main. It should have #62 heads. Rated 390 horsepower stock, the highest horsepower rating of any stock Pontiac engine. Rebuild it to stock specs and put it in your car in place of the 389 and you'll be very pleased. Or, port the heads, beef up the rods with some ARP bolts (or new forged rods) and put a nice cam in it and go have some real fun.
With a 4" stroke the 428 will rev more easily than a 455 and with the extra cubes it makes more torque than the 400 and smaller engines.
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'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 400/4-speed Black on Black, mix & match.