Ok well I have done both single pipe out the back and then built my stacks myself. First, make sure that if either you buy a down pipe or just cut the OEM downpipe make sure cut it before it flares out (to make room for the cat and muffler) second flex pipe works ok for striaght pieces but I recommend just using pipe. Secondly if youre looking to save money, I bought all my pieces through napa, their local warehouse should have everything in stock. I got my 3" to 4" reducer through them when I ran the single pipe out the back. I now have a 4" down pipe so I didnt need one as I stayed with 4" stacks. If you look at the my truck in readers rigs, you will see that at the moment I have flexpipe running under the frame, I am going to 'hard' plumb it over the frame in the next few weeks. This is just my $. 02
Russell