That is a crank vent hole, it probably had a down draft tube running down to the bottom of the engine. These were used in earlier engines as a means to vent the crank case fumes out. That system was replaced with the PCV valve system. You will need to hook the PCV valve into the valve cover, and the other end to a vacuum port on the intake mainfold, there's usually a spot towards the rear of the intake. You can then either block off the hole in the back or even use it as a breather vent.
.
If you have further questions on this, please feel free to reply.
If not, please click the Accept button.
Thanks for asking!
p.s. Nice ride!
__________________
If this has helped you please click ACCEPT any BONUS tip is very kind of you. Please leave feedback.

