OVERHEATING

sorry we were all too busy discussing a range of stuff not strickly relevant to your issue.. apologies

sounds like you found a good buiness there

if everything is stock
and you have the same problem with the borrowed radiator
1) do you have the correct pump. the one for a magnum engine is designed to spin the other way. if you just got the standard pump rebuilt fine, if you didn't and got one off rockauto is it the right one.. if you use the wrong one at low speed it can move some coolant but god knows what its doing at higher speed. if new do you have the email for the order or the paperwork from the package worth checking the part number.
2) are there any idications of a head gasket problem? mayonaise in the oil breather glazed plugs etc grey smoke and a wierd smell out the back
3) do you have a transmission cooler in the bottom of you radiator
4) is the last inch of the tail pipe black/wet do you puff smoke at a specific rpm


at freeway RPM something causes your motor to heat up quicly and over burden the system. if all is well at lower rpm and when in the zone at lo rpm where a fan is necessary. In my mind if everything else is all new and good it has to be spark timing or fueling related

OR
the cap is loose leaking and venting at way below the standard 16LB, did it come with the radiator or did you use your old one? or buy a new one. Is it the correct cap. multiple types but 2 lengths of spring under the cap. a short one in a radiator designed for a long one will vent at a very low pressure.
if it was loose, there would be speckles of coolant on the screen or on the underside of the hood just depends on if you run the rubber seal across the back of the hood where it comes close to the plenum
if its venting most of that goes down the vent pipe, if fitted, and drips below the bottom corner of the radiator support, cable tie a can to the radiator support stick the end of the pipe in it and see if it fills. it really shouldn't. no pipe and just a nipple on the side of the filler spout on the radiator see previous comments about coolant getting on the screen.

if you have a mopar style bottle and return system (1970s kit) is the bottle filling and emptying to a massive degree

if you have a full pressure top bottle (modern car set up) ( i.e an aftermarket set up. an extra high pressure plastic vessel, above the height of the top tank and no normal radiator cap) its going to be harder to check.

the cheapest fix is to just slightly bend the tangs on your current cap.. just a little bit, to make the cap really quite hard to do up and take off..... costs nothing if you have some mole grips or pliers and might be the magic bullet you need.

Dave