Craig, that's true. It certainly would be a good fix. In the end, it came down to considering the pros and cons of:
1) The domino effect. 2" here may not seem like much but what if I'm missing something?
2) I'd have a hole in the roof if I kept the slide to the plans.
3) I'd lose 2" of the companionway opening if I re-fit the slide.
It turns out it's not that bad of a curve. It's curing now, with tack welds and five clamps across the opening. Adding a little bit of timber to finish the butt block aft helped round the new piece and will help hold it in place as it cures. It's not perfect. As expected, there are certain areas slightly off. However, it's not so much that a little epoxy won't fix. I'm encouraged but will know tonight when I remove the clamps and can fair the rest of it.