408 possible timing issue

You say it feels like leaning out. Feels like doesn't equal actual lean. What do plugs look like compared to plug dianosis chart. What is total timing without vacuum advance hooked? Usually total timing is 34 to 36, and initial set by best vacuum reading at idle. If initial is high, too much mech adv will get you lean. But should not shut down rpm. Maybe need to hook up to fuel/air meter to get jets right. I have 408 w 870 holley and jet at 76/78.