You have the upgrade 3G alternator there. The base alternator is the 6G. I have a 3G on my 04' 6.8l too and just did a complete rebuild this year. Below is a picture of mine with the 6 rib belt/pulley. You need an impact to remove the pulley on the alternator but it looks to me like a 6 rib pulley will replace the 8 rib with no issue and line up.

 

"You have the upgrade 3G alternator there. The base alternator is the 6G. I have a 3G on my 04' 6.8l too and just did a complete rebuild this year. Below is a picture of mine with the 6 rib belt/pulley. You need an impact to remove the pulley on the alternator but it looks to me like a 6 rib pulley will replace the 8 rib with no issue and line up."

dkf, I was totally ignorant to 3g, 6g...I looked this up for more info and it was clear why the first Amazon one I got was totally wrong despite the website saying "THIS FITS YOUR VEHICLE!"
Do you know if it's 130 amp or are there different amp ratings for 3G alternators?

 

The 3G is a 130amp. The 6G small case (there is a small case and a large case 6G) that is the other option, it is a 110amp. My dads 03' has a 6G small case. I bought the parts for and rebuilt both myself, figuring out which parts you need can be confusing.

Most times parts sites and stores assume you have a 6G and never mention the 3G. Ford could have done us a favor and only used the 3G.

Here is what a 6G small case alternator looks like.(in my dads 03' 6.8l)

 

Of course, your truck is newer but when I put a new alternator on my 1997 F Super Duty with 7.3 PSD, I noticed the difference immediately and swapped out the pulleys.
Several thousand miles and no problems.

 

Just FYI, the aftermarket alternator builders/rebuilders supply the alternators with the 8 rib pulley for many applications on purpose as it is compatible with either 6 or 8 rib drive setups, saves them from having to supply additional part numbers, reduces their costs and increases profit.

 

thanks for the thread fellas, I was explaining the "whole match parts before re-install" to my kids 8 and 5, and found the pulley to be larger (more ribs) than the original. Was thinking it wasn't a problem, but wanted to make sure. My first Alt went over 235,000 miles

 

When I replaced my alternator, the guy at the parts store swapped the pulley's for me. Hopefully it will work out for you.

 

Yeah, I tried it out, it works just fine. The main issue is diameter which is the same, I guess it could have 12 ribs and it'd be fine really