Horsepower can be anywhere from 130 to 400 in the 70s
No, they are comlpletely different animals. With the exception of maybe some random bolts and maybe the thermostat, no parts interchange directly.
They both have pretty equal potential, but the chevy was never offered as a real performance engine so in stock form plan on it being pretty wimpy. The Pontiac 400 was offered as a performance engine, but by the mid 70s it was pretty wimpy too.
Both have equal potential, but the pontiac weighs more, is larger physically, and will cost more to build after about 350 hp.