Speak to "whoever" you're considering using as a source and investigate the use of the OE pulley and fan to keep the belt at OE length.
1978026 should be the correct "as built" fan and 10498550 should be a correct "as built" pulley. A competent reseller should be able to connect you with a tech person to address the issues. The OE belt is actually spec'd at 2037mm which is 80.1969.
You need to watch where you measure a pulley for diameter, what's considered a replacement for the 10498550 is mentioned to be 62mm which is 2.44 - you have your alternator measure it.
Use this to compare and to use as reference points for your measurements. It would be interesting for you to post your measurements vs. those referenced in the link. I've never measured an OE first hand, yours is original so it could be valuable information. Check the pitch dimension mentioned also.
http://store.alternatorparts.com/241...130-cs144.aspx