Get a new 4-rivet mast step, it's a better design than the 2-rivet step all around. Yes, you will need to purchase a new dolphin striker post to go with the step as the old and new are not interchangeable. The dolphin striker rod does not need to be removed to install the new mast step, however it does need to be free to move so that it can be tightened/loosened as necessary for the new step.
The first thing to do when trying to loosen a frozen striker bar is to use penetrating oil like liquid wrench down into where the striker bar passes through the casting. You can also try using a pair of vice grips on the rod to see if you can break it free of the crossbar end cap. If neither of these work, the last thing you can do (that I know of) is to disconnect the striker bar from the striker post. Then knock out one of the end castings from the crossbar (with the striker bar still installed). Since the post is no longer connected to the bar, there should be enough freedom to allow the end cap to move outward. Once the end cap pops out of the crossbar extrusion, clamp your vice grips on the striker bar and spin the end cap to free it from the bar. Then pull out the other endcap. If this doesn't work, I think your only option would be to cut the striker bar and replace that too.
You may also want to check out my previous post about bedding the mast step to the front crossbar.http://www.hobiecat.com/forums/viewtopic.php?f=13&t=42441