I'd shoot the gun as is, or move it down the road, but that's just me.
I'm probably the black sheep here but I could never get my head around the idea of using a bigger bore gun to shoot little loads, by using tube sets, except for clays competition where you use a carrier barrel.
I love .410s, and 28s, so it's not an issue of not liking little bores. But, if I'm going to shoot them I want them in the appropriate gauge gun. Something just seems out of whack to me, to want to shoot a particular gun so badly that you will pay for tube sets and shoot the "wrong" gauge shells in it. If I really wanted to hunt with that Parker, I would....... as is. And, let the hair go with the hide.
Not trying to start a debate, or argument. Just my opinion, and it's only worth what you paid for it. But, since you asked ...........
SRH