I could not find anyone close by to make the 4" mandrel bent exhaust I wanted, so I too pieced it together with single bends.
Donaldson has a good selection of the bigger sizes, I picked a bunch off the shelf at my favorite big truck parts place. Took them home, used what I could, returned what I could not, and ordered the ones I needed. Take them home for another try, now I only need to order one more bend... It's time consuming, but you can make a nice exhaust from pieces parts.
Here is a picture of the almost finished first pipe, I'll finish it and have it ceramic coated so it'll last a good long time, and reduce the radiated heat on the floor and other parts.
http://rides.webshots.com/photo/2397149610080251109IVLELR

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-