10 sec. 318?
If you have $2500 you could go with a gear vendor tailshaft,and you would have a six speed auto.
Not the first thing I would think to do to get in to the tens with a 318.
You need gears to go fast, that's just simple physics. An overdrive will add a good deal of weight and complication (trans tunnel mods, wiring, etc.)
IMO, money would be better spent elsewhere. If you have the 4 speed, you'd prolly want to make sure it's tight, have the right clutch and a lot of gear in the rear. I think
Passon makes a 'slick shift' deal. Or find member 'Outsider' on here and talk to him about 4 speeds and what it takes for them to be fast. In the end, you might be better off with a low gear 904 and good converter.
But if you're bent on staying with the stick, you'll need to learn how to row the car into the tens. And unless your name is Sox or McCandless you're likely not going to be very efficient at it until you have several laps under your belt.
See
this video to get an idea. Go to 1:01.
Just my 2 bits.