I cross-posted it as I look for my favourite experts in both groups.
I'll never do it again... promise.

I have been asked to re and re an eight-foot T&G board
(pine/spruce/hart-to-tell) from in behind some serious crown-moulding.
The board is 10" wide 1-by. It is split vertically, hence the
home-owners desire to replace it with a board salvaged from another part
of the house. Removing the old board, up-to where it goes behind the
crown-moulding isn't a problem... getting the rest out from behind the
moulding, (a combination wood-trim and plaster 6" 45-degree spring angle
contraption), is daunting. The proverbial can-o-worms is just waiting
for me there.

Here's my question: do I leave a stub of, say..16" then attach a block
and whack it downward, hoping the nails will let go via the end-grain or
do I try to sneak between two pieces of the assembled crown-moulding
with a thin Multimaster blade and cut the board closer to the top, then
to slide the new board into the slot. All I need is the 1 1/2" the
little moulding allows me to hide the new seam. Will the Multimaster cut
1" thick soft-wood across the grain? The thin-ness of the blades appeal
to me more so than the rough-and-tumble saws-all, or even a choice of
ryoba or kataba (they don't plunge-cut worth a darn).

Much appreciated. The home-owners are fabulous people. After shooting a
laser-plane across one of the floors, I went under the house and drove
in a bunch of wedges to elevate the deepest parts up to the laser-line
and we decided to accept it as is... no build-ups.
(It had been a while since I swung a 10-pound sledge over-head, but the
physio has certainly paid off. God bless PT's.

r