I did the same as AH1955, got a couple of 3-1/2" ID x 3-1/2" OD elbows and a 3-1/2" ID x 3-1/2 ID connector and a fistfull of clamps and replaced the bottom of my J pipe.
I went the DIY route myself ... except I replaced both elbows ...
Using a sawzall, I cut the old system ... and fitted the new parts with the system installed IN the truck.
Then to hold everything where I wanted it ... I screwed in a couple 1/8 self-tapping screws at every joint. These screws do a good job at holding everything in place so the pipes don't twist or pull apart
Then I pulled the pipe out and welded it all together. I used no clamps at all.
I ground down the welds a bit and put a coat of hi-temp flat black on it. Looks and sounds great.
My total costs was about $65.00 and an afternoon of work.
oddshot
BTW ... before you start a project like this ... its a good idea to find all the rotted and soft spots in the system. There is always more rot then you think hiding under what looks like a good spot from the outside.
You do this by tapping on the pipes with a metal object like a screwdriver or wrench ... a rotted spot will sound dull when you hit it. Also press on the pipe with your fingers and poke around a bit with an ice-pick.
If you don't get all the rot found and cut out ... the whole project could be a bust.
Its kinda like putting a patch on old wineskins.