I'd recommend that you return the phenolic spacer and go with the simplest 4 hole aluminum one that will do the job. A big issue with these engines is whether or not to use heat under the carb and it has been debated a lot here so I won't drag it up again except to say the last thing you want on a street engine is a heat insulator - non metal spacer - under that carb. My 250 gets by fine year round with no carb heat of any kind but I do run an aluminum spacer and what bit of heat it transfers to the mixture I wouldn't want to lose.
You may end up having to run heat no matter what but the aluminum spacer will enhance that as well.
