Diameter Needed for Spindle Extension

What are shaft diameters of the valve and gearbox output? That would be a good starting point, though these will be steel so need to check allowable shear stress of Monel 400. Also check torsional deflection as this might be significant if there are limit switches etc involved.

What is the size of the butterfly valve?

What is the application environment?

What is the velocity and composition of the fluid through the valve?

Is this a triple-offset butterfly valve?

Much more information is needed to help you.

As a last resort,of course,contact the manufacturer of the valve.(Hmmm!Why didn't I think of that?)

http://www.specialmetals.com/assets/smc/documents/alloys/monel/monel-alloy-400.pdf

https://www.translatorscafe.com/unit-converter/en/pressure/35-20/ksi-newton%2Fmillimeter%C2%B2/

1 ksi [ksi] = 6.89475729317831 newton/millimeter² [N/mm²]

This is the sort of challenge that needs to be asked of the friendly local Valve Distributor. Telephone?

...because the Valve Distributor will give more than an <...opinion...>; it will give a "quotation" and, on purchase, a "warranty" and "after-sales service". That's what it is there for.

Where:
D is the shaft diameter
P is the rated horsepower
S.F. is the safety factor chosen
S is the yield strength in torsion
N is the shaft rpm
321,000 is a factor derived from converting horsepower to torque/balancing units

Note the yield strength in torsion is a calculated value from the tensile yield strength according to ABYC P6.

http://www.wbmetals.com/shafting.asp

There are three strength criteria of interest:
1. Ultimate shear--will the shaft break?
2. Yield (plastic) shear--will the shaft twist enough to take on a permanent incorrect set?
3. Elastic shear--will the shaft twist enough to affect angular indication at the far end?

Codemaster's post reflects 1 and 3; Solar Eagle's reflects 2.