If the 3 pitch is 7 tooth and 3.5 is 6 tooth the circumference will be the same. 7x3=21 and 6x3.5=21
The 3.0 is a 7 tooth and the 3.5 is a 6 tooth.
The 3.0 has a 50.29 constant. The 3.5 has a 50.30 constant. So a 21-51 in either gearcase has the same mph regardless of 3.0 or 3.5 pitch.
https://www.ibackshift.com/calculator