Can I stimulate new growth by removing the dead twigs, or would I
then just have bare trunks?
My guess is that you'll just get bare trunks, Leylandii only grow at
the tips I think. They're very difficult to prune drastically without
getting dead/bare chunks.

The previous owner at our current gaff planted a Castlewellan Gold where a
gate post should have been. Now 20 feet tall, and spreading like mad,
blocking the drive.

But I've found that hacking off 3 or 4 inches in width every few weeks in
the Summer months seems to encourage it to 'green up' in the middle, so no
hideous bare chunks.

Regarding heights, I find that lopping the top leaders off makes leylandii
bush out rather than die. And one newer Castlewellan Gold (I'll never
learn) that I bought ages ago is still growing upwards despite being
heavily topped serveral years ago.

Leylandii are like dangerous dogs. They're OK if you keep them under
control.

Al.