Beautiful truck Joe.
Your math is correct. Your speedo is reading 125% of actual, you need to reduce by 25%.
Since you have a T5, this is a simple and inexpensive fix. Just pull the driven gear out of the tranny (disconnect the speedo cable from tranny, undo the holder, and it'll slide right out. Call the speedometershop guys in the link above and they'll tell you what you need. Should be under $15 bucks.
