Your book looks very well used
Indeed, for free air cooling a little looser fin spacing is better, though I would see 0.1" open air spacing and 0.05" fin thicknes as a better solution. You need to get all the cooling area you can get on that little engine.

(Behind a prop, controlled flow and jacketed cylinder, fin spacing of 0.04" (1mm) works best according to NACA studies)