I was trying to state that it could be done with a decal/overlay. A needle return spring and a counter weight provide some adjustment. No it wont be dead on accurate but then they never were and surely aren't now at 40+ yrs old.
I swapped out my 120 MPH speedo for a 150 MPH in my '69 Barracuda. I bought the 150 MPH speedo on ebay for $75 and paid $165 to get it refurbished (I think I sent it to Redline). Anyway, the 120 speedo didn't have the trip odometer but the 150 did. I removed the 120 speedo and located the trip odometer shaft on the dash. Then I drilled a very small hole in the black spot (depression) on the instrument panel where the trip shaft would normally be. It came out perfect and worked flawlessly. I don't know what you're going to pay for the guy to redo your speedo face and recalibrate but I didn't think I paid too much for what I ended up with...it came out nice. I even had the speedo guy set the mileage to where my old one was!!
He also repainted the needle for free!! One thing that did throw a monkey wrench into the gears was the fact that I had a push on plastic speedo cable/speedo connection and the 150 MPH speedo was the screw on type so I did have to replace the cable. But as it turns out, I was installing a 2004R in the Cuda around the same time and had to buy a longer cable anyway so I just bought one the correct length with the proper ends . Turns out the majority of Mopar transmissions have the same speedo cable connection on the transmission as many other cars (GM, Ford, etc).
All I had to do is make a trip to O'rielly's and buy a GM speedo cable for $7 and it solved the problem.
Of course with the 2004R my top speed is now over 190 miles an hour so the 150 MPH speedo doesn't quite cut the mustard!!! Where's the nearest salt lake????