2002 LS430 . Considering American Bass 8" DVC (=> 8ohm) replacement but is 5.5" deep. The original is 16ohm (which looks like 12 ohm on meter).
What is the depth of the original and the chamber?

 

5.5 inches may be OK. IIR, the original speaker was roughly 3.5 - 4 inches deep. It's just a foam surround below the speaker that forms the enclosure around the sub, so it no doubt has some give in it, but I'm not sure how much..I won't be back home until tomorrow, but I still have my original ML sub in the basement, and will measure it unless someone else chimes in here...