To get the rpm's down you would need a engine pully around 1.37" if the pumps pully is 3.4"...Is there enough room to go with a larger pulley on the pump? 1.37" is awfully small.
To determine pulley / sheave size start with the "known" engine speed and engine sheave / pulley size.
3.4 sheave.. engine speed 3200 - 3600 RPM depending on who set it up, and " desired RPM 1450"
Divide driver speed by desired speed of driven: (driver = engine..driven=pump)
3200 divided by 1450 = 2.20........(A 2.2 to 1 ratio)
Next multiply 2.2 by the known size of the driver 3.4
2.2 x 3.4= 6.8
So you would need a sheave / pulley with size of @ 6.8.