You can get something like
this
and weld it in place of your accordion bellows looking flex section. threads on here report using 3" but i found that to be way too big. I butt joint welded with 2 3/4. Major pain in the butt, getting the orientation & length correct, lots of putting it in and taking it out. Mine is a cali car which i think makes it harder to fit and un-fit the down pipe. Im all for saving $$ but it may have been worth it to spend the $130.00 and avoid the hassle.