I have a ld9 with a m45 with a 2.5 snout pulley and a 7 inch crank pulley (stock alt pulley)looking for a part number or a belt size that will work with this setup?
thx....
 
It's as simple as running a piece of string the way the belt would be routed and measuring the length needed.
2001 Olds Alero (LD9)
650 whp / 543 ft-lb
@turboalero
 
 
Most people with that setup run a stock length belt. 
PRND321 Till I DIE
Old Motor: 160whp & 152ft/lbs, 1/4 Mile 15.4 @88.2
M45 + LD9 + 4T40-E, GO GO GO
 
how would a stock size belt work?
 
he means stock gm supercharger belt that came with the kit, not the stock non supercharged belt. 

RIP silver car. You will be missed.
 
 
ok so I can use  a gatorback 4100729 with a 2.5 snout pulley and a 7 inch crank pulley
the bigger belt I have is the one running a 2.9 snout and a stock pulley