Rick,
I think the real problem is not the trucks themselves on the 70M/70MAC and 80 and 90MAC models, but where they are placed on the frame. I also think a swap from the SD70ACe and M-2s either won't work and certainly won't solve what I agree is definitely an appearance problem.
This question came up a while back about the Dash 9s, i.e., could they be retrofitted with the MTH's newer and much better GE "Hi-ad" truck from the AC440s and ES44s. The consensus was no, because of where the trucks/motor assemblies mounted on the frames on the two different models. Nonetheless, I think some doubter (or optimist) contacted MTH and was told it likely wouldn't work. At any rate, I suspect it won't work for the 70 series models for these reasons:
1: Even if you can install the newer steerable truck, the problem remains where the motor mounts to the trucks through the frame. Whether a truck change fixes that is a hit or miss proposition, with high odds for "miss." Each frame is custom designed for that particular model and places the motor accordingly. I glanced at both an SD70M and an ACe and it didn't look to me like the trucks would come out any better given where the motors mount. In short I think the problem is the frame, not the trucks.
2. The ACes and M-2s have a shorter wheel base than the 70Ms, 70MACs and 80 and 90 MACs. Compare two models and you will see this. Thus you will end up with a shorter truck on the older models, which probably will only increase the awkwardness of the look. (I believe the shorter wheel base actually is prototypical; I seem to recall a design change to the trucks introduced for the prototype 70ACe and M-2 series, that included a shorter wheelbase).
3. Related to the truck redesign, I don't think the swap would be prototypical in appearance either. The trucks just look different. The MTH casting for the older steerable trucks, was pretty credible and represented what the prototype 70, 75, 70MAC and 80 and 90MAC series came with. I think the trucks from the ACe or M-2 would be wrong. And the side frames can't be changed because of the wheel base difference.
Around 2006, I think the first MTH diesels released with the 3/2 trucks actually were SD70Ms. These had been delayed for quite some time and included the new trucks. I had assumed that all subsequent models, including the 70, 80 and 90 MACs had the 3/2 trucks. The ACes and M-2s came soon after, but use different trucks and sideframes.
I agree you've identified a real eyesore with these models. The only solution I can think of (short of persuading MTH to re-tool -- we did that with the Dash 9) would be to cut out the motor mount "key hole" in the frame, tool a new sheet of metal to hold the motor and then mount that to the original frame.
I upgraded this SD70M to a fixed pilot and changed it to an SD75. This came with 3/2 trucks. It's a good model, but I too don't like the truck mounting. I just haven't screwed up my courage yet to try to change it....
RM