From Wikipedia: The SD70MAC (colloquially known as a "Mac" or a "Mac 70") is very similar to the SD70M and SD70I (the SD70I's WhisperCab feature was a build-to-order option), the main difference being that the SD70MAC uses AC traction motors. AC motors, while simpler and more reliable than DC motors, require substantially more expensive inverters to generate a variable-frequency AC signal. The majority of SD70MAC models were produced with 4000 horsepower (2,980 kW) EMD Model 710 prime movers, while later-production SD70MACs are rated at 4300 horsepower (3,210 kW) and feature SD45-style flared radiators. They were also offered with a head-end power generation system for passenger trains. (The SD70MAC is no longer produced due to EPA regulations and has been replaced by the SD70ACe.) Over 1500 examples of this locomotive model were produced. Purchasers include Burlington Northern (now BNSF Railway), Conrail (engines now owned by CSX Transportation), CSX, TFM/Kansas City Southern Railway and the Alaska Railroad.
FromAmerican Rails.com: The newest release of the SD70 is the SD70ACe (most in the industry simply call them SD70 “Ace” or “Aces.” Rated at 4,300 hp and carrying AC traction motors, looking similar to the SD80 and SD90 in terms of its flared rear radiator grill, the boxy-nosed unit is meant to comply with the EPA’s latest environmental regulations and since its debut in 2005 has been fairly successful with a few hundred of the units built to date.
Lastly there is also an SD70M-2 design, which looks virtually identical to the SD70ACe, and in reality is in virtually every way except for the name. They are given the “Dash 2” designation simply to differentiate them from the SD70M model but aside from this are identical to their sister SD70ACe. (EMD says the -2 has DC motors and the ACe has AC motors)
Who knows for sure? I wonder if EMD does? EMD doesn't give any history only their current offerings.
I've never payed close atention to how the radiators look (On NS, all we have are spartan cab SD70's), but in looking at the photos, I would not use an SD45 radiator section. To me, it looks like the radiator grilles and fans are the same size as a regular '70, so all one wold have to do is cut out a section from the rear access doors up, glue it back in at an angle, and add styrene pieces to fillin the gaps. The front is straight (unlike the SD45, which is tapered), and the back simply follows the angle of the rear.