Worst "mod" (well, "part"):
The BRASS thermostat that blew the pin out the back and cooked my 390. Brass thermostats are so evil I suspect that they are manufactured in the colon of Satan.
Did I mention I'm bitter?
And on my '64:
Best mod: Hiding it in the garage
Worst: Buying it in the first place!
Ooh don't let your truck hear that.
On second thought there is a worst mod , Meanies rims. I always thought weld wheel made a good product till i got them and found out they were made in Mexico. Now they're peeling on the inside and it probably be on the outside before too long.