Call me stupid but unless the cam lobes were wiped it doesent have anything to do with the exhaust valves not closing.
The cam you have is a good one.With the 4 speed you could go up one size to the xe274 or the xe 275hl if your heads have better than stock flow.
Anything under .510 lift you don't need adjustable rockers.Some may have run more lift with stockers but the largest I have run is the .509 cam with stock non-adjustable rockers.