Factory noticed the same problem so in 1996 they redesigned the BBC to use a machined flat head surface on the valve cover with an O-ring to seal die cast aluminum valve covers (more rigid than stamped steel) to seal the oil leak at the valve covers. They also went to a one piece rear main seal at the back of the motor and a die cast aluminum timing chain cover to prevent drips from the oil pan.
With a Mark IV engine you have all of the drips in the engine design that the engineers included when the motor was first designed in 1961 (the SBC was still using a road draft tube that dumped excess oil on the road and your garage floor when the new and improved BBC was on the drawing board). So oil leaks were not high on their priority list.
The EPA didn't exist before 1963 and it's first edict was for car manufactures to introduce a Positive Crankcase Ventilation system that kept the oil out of the environment. I not only remember the black streak of oil stained pavement visible on old photographs of heavily traveled paved roads, but living in Florida since 1957 remember the first rains of summer: when that oil created a infinitely long skid pad. The deep south equivalent of driving on ice.
You can glue the valve covers down with RTV and that will stop a lot of it, but you have to be careful as to the amount you use as RTV on the inside will break off eventually and clog the oil pump pick-up. Better to do as they do at the fire station and put a drip pan under your engine.
I kept three of these drip pans under the engine on my cars that I kept inside my seven bay garage (because if left out side they would evaporate in moonlight despite my having a six foot chain link fence topped with barbed wire). This was because the EPA actually has an investigator that goes around to car garages to see how you handled "hazardous wastes" like engine oil; and the fine for oil on the floor was a lot more than drip pans.
Chevy designed the BBC mark II (aka the 427 Mystery engine) that debuted on February 22, 1963 at Dayton beginning in 1961. The young Cadillac Engine engineer was transfered to a special development program and tasked with beating the Pontaic 421 using a 409 block but keeping in mind the engine's tooling had to be paid for by sales of the engine a medium duty to heavy duty truck (diesel engines under 700 cubes didn't exist then). This engine became the Mark II, basically a 409 stroked to 427 cubes with a modern Mark IV porcupine head on top of the old Mark I block. Ford introduced their 427 Cammer (SOHC) FE engine in 1963 and Chrysler offered a little head improvement to their already existing 426 wedge head raised block stroker engine that they called a Hemi in the 1964; so Pontiac's 421 super duty motor's days were already numbered. Chevy banned factory support for all racing in late 1963 after a number of high speed wrecks due to these new engines pushing stock bodied cars beyond their ability to stay on the track.
The Mark III was a new truck engine based upon the tooling that GM bought from Packard motors to cast a 5.000 bore center block capable of supporting five hundred cubes with out Siamesing the bores. Though there were twenty Mark III BBC blocks cast for evaluation the tooling modifications would cost too much to please the bean counters. The Mark III plus the Beach boys 409 song kept the Mark IV engine out of production until late in 1965. The first six months production run was cast with overly thick cylinder walls because even though the 1965 production run had already started the executive board at GM couldn't make up their mind on building a 409 with the new porcupine heads or the "long" stroke high torque 396 engine. The small bore long stroke torque motor won out and we lost the opportunity for a 500 cube BBC in 1966, getting a new and improved 427 instead.