I run a Fluidampr and had to get the spacer as well. I believe it's a .645" spacer. I called March Performance Pulley's and as soon as I said I had purchased a Fluidampr the guy said "..and so you need the spacer? No problem."
It was like $60 for the spacer!
Just give March a quick call at (734) 729-9070.
E