KC-10 Pram - Fitting Seat Risers

[Lurch]

Y'all have been helpful in the past, so here I am again mining the wisdom lode. I installed the seat risers in the pram I'm building and I'm discovering there's a lot of improvisation involved in boat building. I held the risers up with a couple of scrap ply clamp extensions so I could drill and screw them in place:

http://windsorwoodworks.home.comcast.net/riser_clamp.JPG

The next step is to work on the upper surfaces of the risers to bring them level side-to-side using a rabbet plane and chisels. The constantly changing angle rules out power tools. Is this a necessary step, or am I making this too hard?

Thanks,
Don

[htom]

Go along the risers and mark the bevel showing how much must be removed to level the riser. Remove, plane to the marks, and reinstall.