Thanks to all the great suggestions from the list about what my weird
noise was on hard acceleration only. It had sounded like it was coming
from under the passenger floorboard, and sometimes toward the back of
the car, sometimes more from the front.
I should have figured this out earlier, but the cause of the noise
turned out to be the Tiger's water bottle bracket (the piece welded to
the engine compartment panel in the front). It's amazing how that noise
at no time seemed like it was coming from the engine compartment.The
motor mounts must be cracked or sagging since it turns out that under
full acceleration, with the engine torquing to the right, my A/C
compressor pulley (which is hung from the old stock generator bracket
and to the engine) was just making contact with the edge of the water
bottle bracket. Under close inspection, two groves were worn into the
bracket, exactly lining up with the compressor pulley edges.
As a temporary fix before I get the motor mounts replaced I used a
Dremel grinder to take off about 1/3" at the end of the water bottle
bracket. The noise is completely gone now even after repeated full pedal
to the metal runs. Now that I've gotten rid of my pinging (had to retard
the timing more than I like, though) and the bracket noise, and can put
the pedal all the way down again without worrying, all I need is about
100 more horepower to make me happy.