Dot dab planed to interior stone wall?

[Draughtsman]

As a top rail for box skirting, there's a 44x21 sat on the joists for the skirting btm? I was going to set the top rail, then push some plasterboard adhesive down behind ir.
I'd gun some mastic grout down behind the top rail but it stinks so much, I don't know that the smell would go away.
Does dot'n'dab contract much when setting. Is it a solid fix. The wall isn't a great job and the boards don't go right up to it anyway, so there are a few issues I'm trying to tie up here.

 

[Wayners]

Adhesive don't shrink when set, but wood can shrink if wet and dries out

Foam or polymer grab are other adhesive options

 

[lostinthelight]

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[Draughtsman]

Rattled out mortar - mastic grout batten, repair plated shiplaps together (after scribing/planing shiplap downstands) - screwed scrap board to skirting front to brace straight, rolled out stainless builders band - Gorilla glued it 2mm proud of top of skirting (LOTS of pegs), trimmed one leg off skim corner beads - glued to rear of stainless band. Nailed skirting to batten, pack cemented backing skim bead to stonework in one pass (underflush) - let set, packed dryish cement mix flush to top of stainless band to finish

. Paint. Not doing that again.