03 GMC Savana conversion van 5.3 engine.want a spare drive belt but what alternator?
105amp or 145amp? I think the 145amp has a larger case which makes sense to have a longer drive belt. can I decode the vin# to find which alt. my van has?
or can I look at aternator for IIRC this era van was a 244 or something for the 145amp. or can I mearsure the pulley. I'd prefer to not remove the belt just to find the correct length. please advise an thanks

 

Look for the RPO code sticker in the door frame (passenger side I think)

 

I’m not sure but I think the pulley on both alternators are the same…so the belt should be the same…not 100% though

 

For the first belt replacement I buy both belts. Use the one that fits, write it's part number and length in the vehicle log, or in sharpie underside of the hood, and return the one I didn't need. After that I just check my notes.

BTW: Before removing the belt draw out all the pulley, idler, and tensioner locations - AND belt route around them in sharpie or log book.

In your situation - install the new belt now and keep the old belt as the spare. Then you can be Positive it will fit when you need it. <g>

 

For what it's worth I have the 145a. Replacement is 2mm shorter than factory and almost too tight to fit.