!987 Cougar with idle issues

!987 Cougar with 302 high output engine put in around 2013, has never idled correctly. I have had plugs and wires replaced, vacuum leaks checked. Twice put in an EGR valve. Don’t know if it’s a computer issue when we replaced the engine. Previous engine was just regular 302. This one has 302 roller cam engine. If I adjust the idle higher it runs great but idles too fast so it’s hard to brake. Not good for the transmission. If I turn idle down it kills at a stop sign. It’s baffling