I got a V6 housing so I can build up a new rear differential.

Anyway there's a hole on top

What goes in the hole?

 

Abs sensor.

 

as stated above, its the abs sensor plug hole. no biggie, just grab a freeze plug from auto zone and tap it on. its what most people do (including myself). i cant remember 100 percent but i think it was 3/4", best to measure first.