A 304 "might" weigh 10lbs. More than a comparable 266. There were variations in the actual intake manifold castings made over the years that adds and subtracts weight, along with various oil filer setups, etc.
These engines are truck engines bubba, super-heavy-duty truck engines. Massive castings for rigidity when run at max output 24/7/365. They were never produced in "thin-wall" casting technology such as the typical deetroit passenger car iron. No need for "four bolt mains" as they were super-rigid to begin with and did not (will not) turn enough rpm to need that design upgrade. And all internal engine components were manufactured using much higher quality materials than typical passenger car engines.
Ford, GM, and chrysler all built similar duty engines for their truck/stationary/marine applications. A "typical" 440 chrysler used in marine/industrial/motorhome use weighs 150lbs. More than a 440 used in a passenger car.
A chev 454 truck/marine/industrial/motorhome engine assembly likewise weighs about 125lbs. More than the same "size" engine used in a dam pickup.
Ihc made only truck/industrial engineered engines. The "light duty" engines were the amc-supplied stuff such as the 232/258 inline sixes and the 401 v8, those were "bought out" and not ihc-produced and designed engines.