If the car has a TH400 then there should be
JM as the last two digits on the engine pad.
If the car has a 4 speed then there should be
KE as the last two digits on the engine pad.
Example below is a Corvette 427/390 HP with manual trans (IL)
EDIT: It is not that difficult to restamp an engine BTW once it has been decked. So even if you see a JM or KE, without bulletproof documentation there is no way to prove your car has/had an L89 in it.