[Mike_The_Grad]

You said that with your truck idling in gear the RPM's were at 700, and when you put the truck into park the RPM's jumped to 1500? If that is true, something with your timing is off. If you are running a Vacuum Advance canister on your distributor, what vacuum source are you using to supply it? If it's connected to the carburetor, is it a ported vacuum or full manifold vacuum source? These things can effect your engine's timing. Also, if you have vacuum leaks they can cause a lean Air/Fuel mixture ratio which leads to higher combustion chamber temps and higher coolant temps.

There shouldn't be such a large change in RPM's between Park or Neutral and the drive gears. Ideally, it should change less than a couple hundred RPM's.

I'm mostly familiar with small blocks and not sure if the same applies to big blocks though. Each engine and transmission setup is different especially when larger than stock cams, ignition types, and accessories are involved.