You should take the downpipe for the 360 mag manifold side and make copies. I bet you'd sell one per manifold sold. Or even folks that got there own manifold might buy one.
I do make those and sell them, with the manifolds and also sell them as singles. The 360 bell shaped down pipe isn't sold anywhere that I know of. The "factory" salvage yard down pipes are swedged down just behind the flare so they aren't any good for flow. The factory down pipes are also made of smaller pipe. So I buy sections of 2 1/4" pipe and have the pipe flared at a muffler shop. Then, since no muffler shops have the "factory" flaring dies, I take the flared pipe home and hammer (peen) the straight flare into a bell flare (just like the factory) using a 360 magnum manifold as a die form. I also sell the flange adapter which is hard to find. It's hardly worth all the the trouble for what I sell them for but......people need them so I make and sell them, just like the 340, 120 degree down pipe.
In these pics you can see the factory (rusty) bell flared pipe. It's made out of much smaller pipe then it's necked down again making it unusable for good flow.