I mounting the back plate on the k planes and have a gap do to the transom has a slight curved not flat would should use to fill the gap. Any suggestions would be helpful.

Same sealant your using to bond the back plate. Just put on a little heavy in the said area. As long as it squeezes out when tighten fasteners you’ll be fine. 3m 4200 or 5200. 5200 will more than likely pull the gel coat if you ever separate but is double the strength.

I use 3M 4000 for things I want to take apart if necessary. Still a fantastic bond, just not as aggressive as 4200 or 5200.

How big is the curve ? Are you talking about the radius from hull bottom to transom ?
kplanes shouldn't be mounted flush with the hull bottom, the tab/plane should be 3/4 to an inch higher than the hull bottom,
that would have the mounting plate above that radius, (if that's what your referring to). Snap a pic and post it.