The open chamber heads were circular in shape and closed were oval (bath tub
shape). The open chamber ones breath better.
The better Big Block Heads are cast with "HD" on them near the casting
numbers as I remember. These are the only good Big Block Heads . All the
rest are junk or nothing to write home about. (I'm sure someone will respond
and say they are running truck high deck heads and running in the 8.0's)
Since we live in the era of "the money has all gone to it's rightful owners"
or "Barrett Jackson". The heads were expensive 15 years ago. Now in the new
era they are almost priceless museum pieces.
You know after looking at this stuff for 25+ years. GM made the big and
small block the same way. The basic and slightly high po engines (big and
small) all had regular parts. Then there was the best, high performance
engines that were like racing grade. These parts were not as plentiful as
the regular parts.
Example the 1962 327/340hp had all the high performance parts and it had HP.
Funny the 1968 -69 Camera Z-28 engine was almost the same engine. Funny how
they both had a lot of balls and funny there both had almost the same parts
(most expensive/highest HP). The LT-1 was very similar too. It all boiled
down to that one HP intake, HP cam, Double hump heads, forged pistons, HD
crank, solid lifters. A friend said to me 10 years ago and I doubted him,
"it will cost you $4,000 to rebuild one of those engines)
My friends 68 Corvette 427/435 had all the engine castings cast with "HP"
Example, my friend had a 1967 Corvette conv't. Number didn't match 307 cid
engine. He had a holley 4 barrel. 3:25 axle 4 speed. He couldn't light the
tires up off the line in 1st. Then his neighbor gave him a 327/365 HP holley
intake. (I thought a 327/365 was a Fuel injected engine). After installing
the manifold he could light up the tires all day in 1st if he wanted. Look
at what the manifold is worth if you can find one today.
The days of finding any of this stuff for $25 bucks (Bloomington in the
1970's ) is over. In it's day the HP Gm stuff was better than any edelbrock,
mickey thompson, holley after market stuff.
Don't forget, this stuff was made back in the old days of GM, before GM
decided to have it head up it's ass.
You don't get nothing for nothing anymore.