What type of guitar body shape gives that warm, soft sound Jacklin uses?

A slope-shouldered dreadnought like the Gibson J-45 is your best bet — the rounder upper bout and shorter scale produce a warmer, softer attack than square-shoulder dreadnoughts. Mahogany back and sides help too. If you can’t try one, a shorter-scale (24.75" or less) all-mahogany or hog-topped guitar gets you 90% of the way there.