89 Firebird Timing/Rough Idle

My son has an 89 Firebird, 2.8L V6, MPFI, 168,000 miles, which he bought from a local individual. We've gone through the car and found that it idled rough, was running quite rich, and several other problems. We've replaced the timing chain, water pump, radiator, plugs, wire, distributor cap and rotor, TPS sensor, and expansion tank.

The car when started cold idles roughly and wants to die, and if you give too much gas, it dies. After it's been warmed up, it idles fine, just fast and just a slight hesitation when given gas. I wanted to set the timing but my 1/2 or 9/16 distributor wrench doesn't seem to fit on the distributor hold down. Does anyone know what size that nut is?

Also, what else could be causing the engine to run rough at idle as well as rich from the exhaust?

Thank you all in advance, this is a great forum.