Doesn’t quite reach, but it turns out that Lectric Limited carries extra-long negative cables for just such an occasion so order placed
Today I finally installed an NOS temp sender that I’ve had on a shelf for a while, correct for the console gauges, and crimped a ring terminal onto the existing wire to replace the clip-on connector from the old stud-style sender. Swapped the upper radiator hose around while I had the water out - slowly knocking down projects. Gotta clean those plug heat shields!