70duster 360 gets hotter the faster i go

One thing you didn't mention is whether the core support was modified to uncover the 26" radiator. A '70 should have only had a 22" opening if I recall correctly.

It doesn't seem like much but that's 4" blocked off unless it's been modified. Also, you need to keep in mind that with two oil coolers mounted in front of the radiator, all that heat shed by those coolers is going through the radiator, adding to it's job. I know you got fans in there but you may rethink your set-up. Possibly have the fans on the front pushing and the coolers on the engine side of the radiator.