Any reputable speedo shop can correct that problem at a reasonable price.
I'm just sayin' you're not likely to find a Speedo guy on this forum. Even if you do, you're on your back , then your streched out over the engine bay messing with stuff you obviously know nothing about. (It's a 2 piece speedo cable)
I just had a 140 Canadian KMH speedo converted to 140MPH and installed on my car with a one piece cable for less that $225. (Not including the Canadian speedo) Total was less than $400. This stuff looks absolutely factory.
The guy even set up my odometer to within 20 miles of the original and gave me documentation to show the State and any potential buyer (like I'll ever sell it....) that all of it was done correctly and legally.
I'm not questioning your skills, I'm just stating a fact.
Go pay somebody $75 bucks and lose the headache. Why do Buick guys always make everything such a chore?