It would likely be easier/cheaper just to purchase an extended tube from someone like a few of the banner advertisers.
The shell stops in those tube is "stretched metal" and very light metal at that. Getting rid of it may cause other issues like trying to keep it all straight.
I'd be tempted to find a round steel ball the proper diameter and tap
it in lightly and then use a rubber mallet on the outside of the tube.
Rotate between tapping the ball in a bit at a time while encouraging the outside
of the receiver tube.
Maybe using a heat gun to help the steel molecules co-operate hence forth.
Patience and you may succeed.
Something similar......if you have a lathe or know someone with a lathe you could possible make a tapered slug that you can just hammer through the tube.
I will settle for pushing the groove back out far enough to let the follower past, I don't expect to get it back to original, but buying another tube is not going to happen, this is a 30 year old pump that never sold in large numbers, and in zero numbers in North America. The damage to the metal isn't reversible regrettably, but like I say, I just need the metal moving back out enough to let the follower pass.