There seems to be some confusion about the different head types used on Chevy Rat motors. Some people think there are only "oval" or "rectangular" ports, and some people don't understand the deal with "open" vs. "closed" chamber heads. This article, taken from the book by Ed Staffel "How To Build Max Performance Chevy Rat Motors", should help to explain the differences.
Small Oval Port (i.e. "peanut" port) heads
"Mark IV, Gen V and Gen VI small oval port cast iron heads, commonly called "truck", "round" or "peanut" ports and made from 1976 on, make power and high torque at very low RPM's and are helpful when the motor is pulling heavy loads in trucks, trailers or RV's. The velocity of air/fuel mixture in these heads is very high at low engine speeds, however they run out of breath at RPMs above 5,000 and are not really suitable for street performance or race applications, although larger valves, bowl and other port work can improve the flow situation. Small oval port heads have intake valves that are 2.06" in size, with 1.72" exhaust valves. They have hardened valve seats. These heads can have closed or open chambers (small or large chambers respectively). Most are found on trucks including late models, but there were some full size passenger cars and station wagons in the mid-70s that could be ordered with a 454 and may have the small oval port heads."
Large Oval Port Heads
"Mark IV large oval port heads make their power over a broader and higher RPM range. They are found on cars and trucks from 1965 through 1990 and make excellent street performance heads. They can be used on bracket drag race motors, after bowl porting and installation of bigger valves, with great results because they make a large amount of torque and will breathe well to about 6500RPM. They come with 2.06" intake and 1.72" exhaust valves. They have closed or open combustion chambers, and depending on when they were made have either soft or hardened valve seats. The open chamber style castings will flow more air than the closed chamber designs. This is due mainly to less valve shrouding. The downside with open chamber heads is that you will have to run a dome top piston to get any level of compression ratio above 8.5:1.
A good starting place would be cast iron, large oval port, open chamber casting #'s '781', '049', '359' or '241'. In 1996, Chevy introduced an over the counter aluminum open chamber head with large oval ports. This is the first factory big block aluminum head with large oval ports. This head will fit and seal with Mark IV, Gen V and Gen VI blocks.
Rectangular Port Heads
"Rectangular or 'square' port cast iron and aluminum cylinder heads are strictly for high-performance use. Because the intake port dimensions are so large, air/fuel velocities are at a low speed in the ports until the RPM's of the motor get high. Then the power comes on like gangbusters. Applications include high performance street use and racing where large volumes of air and fuel are required at high RPMs. Square port heads came with larger 2.19" intake and 1.88" exhaust valves. Some early closed chamber heads had 1.84" or 1.72" exhaust valves. Open or closed chambers were available with soft or hard valve seats. Once again, the open chamber designs on production heads will flow more air than the closed chamber production variety. If your engine is going to operate in the 7,000+ RPM range or if you are building a large cubic inch race motor, then the rec. port heads will make the power you need, but at a higher RPM."
