I know nothing about how horsepower is measured or was measured but the 53-55 (and early 1955?) Corvette 235 engines supposedly produced 150 hp (1958 235 engines supposedly produced 145 hp). The Corvette 235 had three side-draft carburetors and a modified lift (261 truck) camshaft.

The GMC 302 straight six with Wayne "12 port" head supposedly produced 300+ hp.

Here is another interesting article about 235s.