You need to change the idler pulley directly below the blower snout to a 90mm. You can buy a bling pulley online for big bucks or you can go buy a generic idler pulley from the corner auto parts store.
One of the guys over on fnsweet forum posted a thread a while back with all the M90 belt sizes and part numbers for different years. For most of them Roush is your only source because they are an odd size. I bought a spare and keep it in my trunk just because a broken belt could put my car down for a week.
That's me.
Here...have fun, guys:
Roush 6 rib belts:
2005/2006 FEAD M90
belt: RPP# R07020015 -13-AA / PN 401967 / 131.732"
2007/2008 FEAD M90
belt: RPP# R07020032-13-AA / PN 402065 / 132.441"
2009/2010 FEAD M90
belt: RPP# R07020063-13-AA / PN 404078 / 132.638"
2007/2008 TVS / P51 belt: RPP# R07020041-13-AA / PN 403805 / 134.370" / (135" OC)*
There is also another 6 rib TVS single belt Phase Two belt that came out PN 421160 3378MM / 132.992”
Roush P51B Dual Belt:
1st Sheave: PN 404168 Stock Mustang GT Belt
2nd Sheave: PN 404306 1670MM / 65.748”
Goodyear 8 Rib Gatorback belts (no 6 ribs available in Roush lengths from Goodyear):
4081295 8PK3290 8 129.5 130.25
4081360 8PK3455 8 136 136.75
DAYCO 8 rib belts (Top width 1.10” .169” thick) (No 6 ribs in Roush length from DAYCO):
[FONT="]5081325 132.50 8 W Profile EPDM, Aramid Reinforced Rubber [/FONT]
[FONT="]5081363 136.30 8 W Profile EPDM, Aramid Reinforced Rubber [/FONT]
[FONT="]GATES belts (6 and 8 rib):[/FONT]
[FONT="]K061360 [/FONT][FONT="]136 5/8 6 Micro V belt Width 13/16 (20mm) 40°rib angle[/FONT]
[FONT="]K081331 [/FONT][FONT="]133 3/4 8 Micro V belt Width 1 3/32 (28MM) 40°rib angle[/FONT]
[FONT="]K061360HD[/FONT][FONT="] 136 1/2 6 13/16 (20MM) 40° FLEET runner Heavy Duty[/FONT]
[FONT="]K081331HD[/FONT][FONT="] 133 ¾ 8 1 3/32 (28) 40° FLEET runner Heavy Duty[/FONT]
Keep in mind that with an X-Acto knife and some patience, you can easily cut a 10 or 8 rib belt down to a 6 rib by removing the extra unwanted ribs. Like this:
OP...you were given the easy answer - you need an 85 or 90MM idler immediately BELOW the supercharger snout pulley. Autozone, O'Reilley's, Schuck's, Pep Boys, Advance Auto etc. will all have what you need for cheap.