Hi Paul
I would definitely smooth it out and have a smooth rifle. One reason this is important, is that if you shoot a few shots through it, depending on the condition of the rifling, you will find a lot of leading in the grooves of the barrel. I think it would be nearly impossible to keep it clean. As far as shot spread, once it is a smooth-bore, I think it would be about the same as any un-choked barrel. No idea how the rifling would affect your pattern.
Is the barrel straight? I assume it is, since it is a Spanish-made reproduction. What is the distance across the flats? Once that is known, and you do chosen to re-bore it, you might even be able to open it up a little more, if that is desirable to you. If you have an inch across the exterior flats, then boring it out to .50 cal would be totally possible, and maybe more. I would want to stay with standard calibers, since you might want to try shooting buck-and-ball loads through it as well.
Matt