RE: Evolution 60 NX in a 40 size case...
I looked at Horizon Hobby's web page on the EVO engines. I found that the EVO 60NX had essentially the same foot print as the 46, but weighed in 16.32 OZ (Engine only, with muffler or pipe much heavier). The .46 weighhed less. Did I read something wrong? Do you have a special engine? I think you need to go back and look at the weights and reread the hype. Horizon seems to indicate the engines fit in the same bolt pattern meaning a bigger more powerful engine fitting into a smaller location, not a weight reduction.
I would not worry to much about butt weight since you are going up in power considerably. An ounce or two in the butt, set way back, say just forward of the verticle (rudder) can easily compensate for the additional weight. The 60 is volumetrically 30% more capacity than a 46. Without much thought that seems to indicate a considerable greater prop and similar RPMs ~ a lot more power!
Don't know what you are doing with the engine, the kind of airplane, so it is hard to draw any final conclusions. But in general a 60 size engine is gonna make a 40 sized airplane really zip along. An ounce or two added to a 5 pound plane is not much, maybe a 3 to 5% weight gain. That is not a big trade off when you are increasing the power 30% and the weight by 5%. Its gonna rip.
Cheers,
Chip