It's very likely I'll take these off and find they're still just fine. Adding the custom-formed backing block and drilling/positioning the 3 flange holes exactly right the first time, adds quite a bit of labor and expense. I figure it's several days of labor (minimum) and 2-4x the materials cost, depending on how I go about it.

If I just replace like-for-like, it's an easy job and I should be able to do 4 thru-hulls and ball valves in half a day (if all goes without a hitch). A second half-day in the worst case, if I run into a snag. The more I think about it, I like this idea better. I'll likely use Marelon thru-hulls with the Marelon ball valves because the thread tapers will match (vs. straight thread thru-hulls and tapered thread ball valves). Otherwise I'd be fine with Bronze if I can match the threads. The next time they need attention it will likely be someone else's job.