I'm not the final authority, but as far as I know, exhaust-wise there is no difference between a 340 and 360. There might be a difference in headers, if your 360 has something other than Mopar iron heads. That wouldn't affect what we are discussing here. The only drawback would be if you thought you needed 3" at some point. My guesss would be that you'd have to be over 500 hp before 2-1/2" became a stumbling block.
I am satisfied with my Magnaflow, if I had to do it over I would do the same thing.
One thing I did do, the kit directions said to do some welding and I'm not set up to weld stainless so I got extra clamps. 'Here', the local International big truck dealer has wide stainless steel band clamps in any size from 2-1/2" on up for only about $6 each, I got a couple more clamps from there and my system is 100% clamped-and all with band clamps so the pipes are not kinked or dented, if I ever want to take it off it should all slide apart.