Missing at cruise. 5

Just pulled my 340 and set in the new 408. Engine runs well and is stout but I have a random miss at cruise rpm. About 3000. Same carb and distributor as the 340 which didn't do this. Initial is at 24 and 32 total and 50 with vacuum advance hooked up to timed port on carb. If I unhook the VA it almost goes away. Is 50 to much. Soon as you lightly accelerate it goes away. Carb is a out of the box proform 750 dp. I think I might be lean at cruise. This is a 10:1 engine with ported sm heads and a solid 254 260 at .050 cam. 3.91 gears.