What do you have for accessories? Transmission? Is the big block complete with everything?
The big block conversion is gonna cost more if you don’t have all the parts. New radiator, new transmission, plus the rebuild. It shouldn’t overheat any more than anything else, horsepower is heat. If anything a 400 hp 440 should actually run cooler than a 400hp small block, the small block has to be a bit higher strung. Neither should be hard to cool though.
As for what it was originally, neither engine is stock so it really doesn’t matter beyond the expense of the parts you need for the conversion.
You can make plenty of power with a small block, more than the factory ever did with the big blocks and pretty easily too. Big old heavy engine up front just makes it more of a barge in the corners.