Shock spring length

Not universal.

Nothing is "standard" between suspension brands, except maybe axles.  Usually the triple clamps are different, linkage is different, internals, spring diameters/lengths, etc.

You may not be able to assemble the shock with that longer spring unless you compress it first.  Then you will have a huge amount of preload.  Then, whats the point?  Ride it with the correct spring or buy the one that fits.

If there’s enough threads to get to the correct sag point, spring length should not matter. As long as you are measuring sag at the axle.

There's a  variety of dimensions in Shock springs.

ID - and it can differ end to end, OD, Free Length, Travel available within the spring itself. Note : Shock vs Spring Travel is Incredibly important! Coil Binding a spring Before the shock and it's bottoming elements have reached full travel, is a Brutal Impact. It destroys Shocks / Frames / Swingarms, and, possibly the rider, when a catastrophic failure occurs because of it. 

For me to put the type of spring I wanted on my PDS CR conversion, I had to make a body pre-load adjuster spacer, to accommodate the new springs larger ID at that end of the spring (and to allow use of a thrust bearing set up I made) and a new shaft eye end spring clip / plate for both the ID and OD change in the springs, and with a 'staged back' spring landing, to accommodate the far longer spring ( even with being the Ohlins shock having spring pre-load threads All the way back to the body eye) than the original Ohlins spring.

Some pictures of the shaft eye clip / plate - can't find my pics of the body end adaptor I made - original Ohlins to the left, my Bear Built ones to the right :

The stiffer spring is shorter by nature. As was stated if it fit's the shock body and spacers your GTG.