They were only available on the high performance solid lifter rectangular intake port HiPerf engines. The Corvette differed from passenger cars by having a sloping drivers side cover to clear the power brake booster on install or removal to adjust valves. All Pass engines received painted covers. Also the HiPerf engine had grooved rocker balls and long slot (stamped H) rocker arms while the Pass cars had smooth rocker balls.
Chromed or painted all BBC covers had oil drippers under the cover up until 1972 when they went away on Pass engines and a year later on HiPerf engines. You really want a reproduction cover with oil drippers with a modern cam as they are as aggressive as the solid lifter cams of the early sixties.
BBC also all used to have 7/16th inch diameter push rods up until 1972, after that they were all 3/8th inch in diameter like the earlier SBC used (after 1972 the SBC went to 5/16th inch diameter push rods as smog cams entered the production line). To use a 7/16th inch push rod on a modern motor you have to replace the guide plate for the push rod as well.
The only other thing to add is a painted cover will resist oil leaks better than a chromed one as the cork gaskets won't seal against the polished chrome plating. That is why the factory went to die cast aluminum valve covers with the Gen V and Gen IV engines. They used an O-ring to seal instead of a cork gasket.
Since only a "real" SS Camaro, Chevelle, or Nova with a HiPerf BBC under the hood had chrome valve covers the aftermarket (Cal Custom, M/T, Edelbrock, or Chevrolet's service department selling Corvette or Chevrolet finned die cast covers where as common as painted covers back in the sixties. It was like mag wheels and Hurst shifters, everybody couldn't wait to upgrade their ride from stock with aftermarket parts found in car mags of the sixties.
HalnasLT1 - Dave must have been having an off-day when he responded to your note from earlier this week.
Chrome valve covers were standard issue from the factory on all mid to late 60's big block B body's when the Super Sport option was selected. Also, if the (B body) car was equipped with power brakes, the drivers side valve cover had the indentation in the back. You'd never get the valve cover off if it wasn't there.
Also, I've removed plenty of big block valve covers from 396 cars of the same era that had no drippers spot welded into the valve cover. Fact is, they're not really needed anymore and often interfere with rocker nuts or poly locks. Most modern rocker arm manufacturers put groves or slots into the rocker arm balls which hold oil unlike the factory balls that were smooth across the contact surface. Use the rocker balls with slots and you don't need the drippers.
As for pushrods, from what I'm recalling, all hydraulic cam big blocks of this era used 3/8" pushrods. 7/16" diameter pushrods were mainly used in the solid lifter high performance engines.
As far as getting a good oil seal when using a chrome valve cover, it's not problem these days. Just use a thin film of a high quality silicone sealant between the valve cover and the gasket. Been doing this for years with great success and it doesn't matter what type of gasket your using.