I love both of those names, especially
Alasdair!
Linden I would keep spelled just like that. That's how the tree is spelled, and that way it won't be so closely associated with LBJ. I think it's a great first name because you could call him Lyn/Lin, which I love.
Alasdair is the spelling I most prefer, but in some instances I think
Alistair looks better.
Alastar is my least favorite just because the word star sticks out strangely.