How much travel do the axle boxes have within the hornblocks? If you want to use all ,of this distance, you cannot have springs which are chock before this.

How many turns do the springs have? This will determine the sold length of the spring (Turns x 0.048 )

If this is more than your 7/8 space, the springs are unsuitable.

Shortening a spring by cutting off turns will increase the rate, but increase the stress in the spring, possibly to point where it will fail when repeatedly fully loaded.

You need to check the rate of your springs.

18SWG is 0.048 diameter, so allowing 1/4" travel the spring would be solid in the remaining 5/8, if it had 13 turns..

Really, the spring rate (lbs / inch ) will be determined by the amount of travel that you want, given the number of springs to carry the weight of the loco, for a given deflection of axle boxes.

Say you have 8 springs ( 2 per axlebox on an 0-4-0 ) carrying a loco weighing 112 lbs, (to make the arithmetic easy ), and your desired axlebox travel is 1/4"; .so each spring needs to have a rate of 56 lbs / inch.

HTH

Howard