Mike O'D said:
"The SIV is 1534, which would make the miles read low, but speed would be closer.".
NOTE: Edited at 8:45 AM to fix errors from previous 2:00 AM posting
That would only be true if the speedo part of the 1534 had the same calibration error as your 1000 speedo, which seems likely but is not assured. If the MPH indicator still has the factory calibration it will match the Odometer ratio. Calibration of the MPH indicator depends on magnetization of the spinning magnet and proper adjustment of the hairsprings on the needle pivot.
Assuming your test was accurate, and that your present speedo is a 1000 TPM and has not been changed at some time, the correct speedo for your set up would be 1453 and that would provide accurate miles and accurate speed if it still had factory calibrated MPH. Your present speedo has a precise calibration of 1000 TPM, and should read 58 MPH at 40 MPH (1453/1000) *40 = 58 . But based on your 62 vs 40 test results the MPH indication reads about 7% higher than factory cal, due most likely to aging/damaged/weakened hairsprings.
The 1408 odometer would read about 3% high, and the MPH, if calibrated, would also read about 3% high. But if the hairsprings have weakened similar to your present speedo, it would read about 10% high (3 % low from the factory, plus 7% higher than factory due to aging.
So an SIV 1543 seems an OK choice, It will read 6% low in Odometer but if the aging of the hairprings is the same,which seems likely but not assured, then the MPH will work well.
MPH calibration errors are apparently due to either weakened magnet which causes it to read low, or weakened springs which cause it to read high. I am guessing that springs are more likely to weaken.
I think you will have a hard time finding either of these. So a gearbox looks like a best bet. A gear reduction of 0.645:1 should provide accurate MPH, but your odometer will read about 6 % low