Is there any difference in the block or short block of an early 400 and a later model like a 1975 400, Is all the H.P. difference in the heads and carb?
Dustin

 

earlier modles had higher compression so more power then the late models. And yeah the heads where different. carbs are about the same though.

 

bores and stroke were all the same, heads intakes were different, watch later 400's 78 and up had weaker oil pumps for some reason with a smaller pickup screen

 

As cppainter said, Pontiac did almost all of the power decreases using larger combustion chambers in the heads, and also don't forget about the SAE power rating change. Just check through this page showing the combustion chamber volumes and see what happens once 1971 and later rolled around.

http://www.angelfire.com/fl4/pontiacdude428/HeadID.html