I went to a local tranny shop.
The speedo was 20 mph fast after changing from a 2.73 to a 3.42 gear set in the rear.
The owner of the tranny shop put a driven gear like you are showing with more teeth in the tranny and said "try it out". I had a hand held GPS to show MPH.
Still fast. Drove back and he put another in with more teeth..another drive...still a little fast...another more teeth gear and another drive.
More time to let headers cool off than to change that sucker.
4 mph fast now and the speedo works well. He didn't have any with more teeth and I said the 4 mph fast was just fine.
$20 total cost.
Try out a local long time tranny shop that is private owned and you might get the same service, hopefully.