Hiya
Its surprising that the bracket rotted but the rigger and the A post didn't.
The replacement bracket does indeed slid inside the a post, I replaced both bracket and A post on mine but somone had done something like you need on the other side. It looks like they chopped out the bottom end of the A post, slid in the new one and welded at both the visible join at the bottom and a through weld part way above the bottom door hinge. Definitely a farm repair as no angle grinding in sight.
Make sure you brace everything before you cut, and measure it twice from top corner of the bulkhead to the opposite rigger otherwise you'll have fun making it all line up.
hope that helps