I've shortened several sets of Mantons, just using a good quality hacksaw with a high-carbon steel medium-tooth blade.
To keep the blade straight while cutting, I first use a pipe cutting hand tool to create the initial "ring" around the pushrod.
After a few turns (tightening the adjuster slightly after each turn) you will have a ring to "start" the hacksaw blade. Cut smoothly and block the cut with a vise to prevent slipping.
Clean up the end with a file, blow it out with air and tap in the ball with a pair of old lifters.
It takes about 3-4 minutes per pushrod to finish them nicely