How was the crankcase vented before you replaced the rocker cover filler cap with the breather? If there was no air-in source, then you put the breather on the rocker cover that would partially explain the rough idle. Going from no fresh air-in to a breather would act like a vacuum bleed/leak.

Looks like there is a pipe thread plug right under the hose that connects the brake booster to the carb manifold vacuum port, might be a bear to remove though.

Looks like you have a Holley 4175 spread bore vacuum secondary carb. There should be a power brake vacuum port at the rear of the base plate. These are sometimes plugged with a pipe thread plug. There may be some without but worth a look. Looks to me you have the pcv hooked to stepped vacuum. The PCV should be connected to a 3/8" vacuum port, such as the one you have the brake hooked up to. I'd take that carb off and check for a hidden brake vacuum port at the rear, if there is definitely not one, then tee off the front one you have the brake hose attached to or pull a plug in the manifold and install a nipple for the brake or PCV hose.