If it's a "super sucker type spacer aka blended spacer you want, the phenolic version available from Canton is a better piece than the plastic from hvh...
https://www.cantonracingproducts.com...CER-BLENDED-1/
That said, I have never seen any improvement on the time slip from any spacer on any car I've owned or tuned in the over 20 years I've been racing however I have used 1/2" phenolic or wood to isolate the carb from manifold heat.
Further, many bracket cars are running a manifold with too much plenum volume for optimum performance and the addition of a spacer just makes it worse.