I guess if the OP wants to change something he should ask the machine shop guy. If the metal is 0.125" and the holes are indeed 1/4" we're talking about .278 oz and with a radius of 4.5", we get an imbalance of 1.25 oz-in. The 5.9Mag's imbalance is something like 14.5 oz-in, and the 360 is a little more than 18.

I just found another picture of a stock neutral balance flexplate and it had the same kind of holes in the same place, just not as many.

I'm saying that is a neutral balanced flexplate and can be swapped for any other neutral balanced flywheel or flexplate.

R.