If you have access to it, a side-angle grinder with a flapper sanding disk makes short work of tapering out that repair. The main point is that you don't want to have the sharp 90 degree edge that you would have if you just repair the hull as it sits right now. That would load the seam between old and new almostly completely in shear and there will be minimal contact area to carry the load. You want lots of contact area.

Also be sure to bed the new aluminum plate with some thickened resin (preferably use cabosil filler {fumed silica}) so it makes full contact with the inner skin of the hull and around it's sides. Also coat some thickened resin on the outside of that plate before starting to build up your glass so the plate has very good contact all the way around it with everything it touches. It may work loose if it has gaps around it.

This really shouldn't be a tough repair...the boat is very solid in this area and you can't mess it up to the point that it can't be fixed. Don't sweat it and have at it!