602 and 604 are the last 3 digits of the part number used for the GM circle track crate engines

the 602 is an iron headed version with a cast crank and hydraulic flat tappet camshaft
the 604 is more of a performance oriented engine with aluminum heads, billet crankshaft, and hydraulic roller camshaft

both engines have been proven very durable in circle track use over the last 10 years and the parity is incredible for each individual engine

the 602 is advertised at 350 HP and the 604 is at 400 HP

I will add more info later on tonight
thanks for asking
Brad
www.race-1.com
www.paceperformance.com