Patrick James Fitzwater: I think the dealer sold you the right belt, it is 75-1/2" long.
Did you get a new Mule Drive Extension Spring, P/N IH-489607-R4, shown as Item 5 in the diagram below?
You can find the diagram above in Parts Lookup under "Attachments" for your 128 Garden Tractor; it is the "Hitch, Quick Attach for 38 42 48 Blade SN 400,001 to 529,999"
The belt for the 38" and 42" decks is called out as P/N IH-490489-R2 and it is listed as 80" long.
Or perhaps you have the Hitch as shown below, for tractors SN 218,010 to 400,000:
Some more experienced heads may be able to compare your photographs to the diagrams and tell you which one is on your tractor.
Gerry Ide (and Charlie): Point taken on the deck measurement. I often wondered why my sums didn't always add up to the given deck width, which is why I told James he could get an "idea" of which deck he had. Learned something, thank you.
Edit: I checked your photos and I saw the spring as shown for the later hitch, the first one I posted above, on top. I don't see a spring in the other diagram.