That chart is based on a 13-tooth drive gear, the pre-'66 727 has an 8-tooth drive gear.

Based on the OP's info, a 25.3" tire and a 2.76 gear, he needs a 17.6 tooth pinion so either a 17 or 18 will work but won't be dead-on correct.

If the speedo is off with the correct pinion, use the odometer instead of the speedo to gauge accuracy.