Thanks for the advice on replacing thru-hulls. I've decided to replace the OEM water speed & depth transducers with a single smart sensor that does both (Airmar DST800, comes with the Raymarine ST60 Plus tridata). Haven't determined where I'll place it, but I did look at the installation instructions briefly and saw that one of the recommended locations was forward of the keel stub. I'll know more when I remove all the thru-hulls later this week.
The hull thickness was of interest because I read somewhere that it's not necessary to use a backing block if the hull is 1" thick or more. And the OEM waste outlet thru-hull was indeed installed without a backing block! The thru-hull's not been hit hard in 35 years, so not an issue. At any rate, onward & upward. ~Joel