OK, shot 25-year-old fill valve was removed after much drama (had to
disassemble furniture around toilet; could not budge locknut and had to
use bolt cutter). Installed (finally!) Fluidmaster plastic fill valve,
which went perfectly fine on the last toilet I put one on, but not here.
I can't tighten the dang thing. It seems there's a *heavy* foam
insulation inside the tank and the old (metal) fill valve just got the
tighten-the-hell-out-of-it treatment, making it sink into the foam. But
with this plastic valve and a nylon locknut, I'm afraid to do that,
given the strong DO NOT OVERTIGHTEN remarks in the instructions. I kept
to the strict hand-tighten, plus 1/4 turn with a wrench rule. So what I
have is a drippy fill valve.

Should I just tighten it more, reasoning that the foam will be crushed,
easing any overpressure on the plastic? Or should I cut away part of the
foam, so the plastic can make direct contact with the porcelain as
designed? Or what?

It almost seems inconsequential that the standard oval handle on the
shut-off valve more or less instantly crumbled on being grabbed by a
wrench (had to, it was frozen), and the screw won't come out, so I'll
have to have my brother sweat-solder a new valve in.

Am I the only one here who can contemplate almost any other project with
an even temper, but the very thought of plumbing makes my blood pressure
sizzle?