I’m not an expert but have modified several blades over the years with good results. Based upon this limited experience your plan seems feasible. As you proposed you could compensate for the reduction in height by increasing the width. If you chop 6 inches off the bottom and increase the width by 2.33 inches, you’ll still have a reasonable aspect ratio if all 36” remain immersed. You’ll probably want to move the leading edge forward .6” and extend the trailing edge aft 1.7” to keep the maximum depth of chord in the same location.
Assuming your rudder currently has a foil shape and you wish to retain a fair foil shape, you should refer to NACA foil shapes. A quick Google search came up with is link Foil FAQ
It appears quite useful. As I recall, epoxyworks has a good article on fairing a keel which would give you a good idea what you’re getting into.