a simple muffler question

[Boise69]

Ok, my flowtech muffler rusted out the other day, odd because thats the side where i get a lot of water out of the exhaust and the other muffler is in better shape but thats not the question...

i want to just replace the muffler and keep driving until i'm ready for the complete tear down in the spring... I just want to cut the rusted muffler out an put a new one in. Do i saws all it off and weld in a new one? In a few months i'm going to pull the whole thing out and go with the magnaflow X system but i'm not quite ready yet. If i just want to splice one in for now will i need to weld in a 2.5 pipe or how do i handle this?

thanks