Ken

Try gently prying the siding off. If the nails will not give, slip a hack saw blade up behind the siding and cut each nail. Maybe a long flexable fine-tooth sawzall blade would work but you risk damaging the clapboard if the blade binds up while using the power tool.

Jim

The closer you look the more you see.
"Heavy timber framing is not a lost art" Fred Hodgson, 1909