http://ridetechgarage.com/more/1968-camaro-velocity/
Exhaust is also run under the axle on Bret's Air Ride camaro, but it looks like they are using bigger pipes than I am, maybe 3"? Of course their car is set up to run extremely low, and I think it has air rides early version of a g-link type suspension.
When I think about it, my rear end was setup different than normal for a 12 bolt. It was cut to center the pinion instead of the normal centering of the housing, so the slight offset may be the issue with the tight over the axle clearance I have. Mock it up and check is the only way to be sure.