would it be a waste of money to run 4" exhaust back from ym dp if the dp is 3.5"?

 

also, will i run into problems if i am running a 3.5" dp and my exhaust is i believe a 3?

I dont want to have a whole bunch of flow issues.

Also, what is the advantage of using ss304 pipe over aluminized steel. And, what type of filler rod do i used to weld the two together?

Thanks