[elmariachi]

No two cars are the same, so I wouldn't put a lot of stock when reading that things like a 13 lb cap are "wrong." The smartest investment I made when I was chasing cooling problems on my car was one of the little $50 handheld infrared temp guns from Summit. Take some readings at the intake manifold water exit, the radiator inlet and outlet; you need a solid 30+ degree drop happening in your radiator to adequately cool the beast. My radiator reads around 180-190 at the inlet and about 140-145 at the outlet. This was made possible by the Taurus cooling fan (use search function to find more info).

Your #1 and #2 options are right on, but be sure to check the new t-stat in a pot of water first so you know exactly when its opening and closing (no two are the same and you sometimes have to buy a few to find one that's dead on 180 opening.) Here is Houston where the heat indexes have been at or above 100F I am running a $7 NAPA MOPAR t-stat; the one with the bigger center hole.

You don't mention whether or not you are running a recovery system and this matters on the type of cap. If you aren't expanding and recovering coolant to and from an external tank, the cap needs to be the open, non-recovery type.

If your pulleys are the same size and none of this makes an impact, the you are down to either the fan isn't drawing enough air, the water pump is failing or the radiator could be clogged and full of deposits. But if you take your time you should be able to find hot spots on the radiator face with the little gun.

One last item...retarded timing will drive temps up quick. Might be worth checking that before you dig in too far.

Good luck.