You can increase the length of the butt portion by forging it out. It'll be somewhat thinner as the metal is drawn out.
But carefully shaping back into a slight rounded contour will give it the look needed.
Lacking a 'forge',,You'll need at least an acytelene torch to get enough heat quickly enough before forging it out.
I wouldn't do it cold, but even a small amount may be able to be gotten from it that way. There's a chance it can crack during cold forging though if you over do it.
You can soft solder, hard solder(silver solder) and/or braze the butt plate also. Either steel to steel or brass to steel.
You can weld an extension on also if you have access to that.
Alot of work to cut and place an insert inbetween the halves to increase the length, and have it look good too.
On some brass butt plates, I've seen the toe and the heal tips made of steel. Brazed into place, possibly just silver soldered. The idea is to better take the wear of ground contact during loading and setting around talking!
You could go the opposite way, and make a brass extension at the toe and a corresponding brass tip at the heal after trimming that a bit to keep the contour right.
Kind of defeats the purpose of a 'wear plate' but it'll get you the added length to the plate w/o it looking so much like an add-on piece for no other purpose than that.
Or,,you could just set that plate aside for another project (there will be more!), and purchase another in a size better suited for this particular rifle.
Just some thoughts...
