A friend of mine is going to weld on larger end tanks to the stock intercooler for me, provided I find the 2.5 " piping. He said it's best if I find out what type of aluminum the intercooler is so that I can find the same type to make welding easier. Does anyone know? I did search and did not find anything. Maybe I'm not searching the right terms

 

Any common aluminum will work well. If you must go for some 6061. Either T4 or T6 will do (the difference being their strengths, T6 being superior. In this case either will work.

Just make sure you don't get 6061-O (O=annealed,) or tempered. Wikipedia it or 6061 aluminum alloys for more information.