well I saw a thread recently where a guy wanted to put an ls1 in a truck and was looking for longtube headers but they only made headers or BBC swapped trucks. He was trying to figure out if you could modify BBC heaers to fit on an ls1.
In your situation obviously the opposite would be beneficiary as there are a couple companies that make headers for ls1 swapped foxes. That would be a good start but still no assurances that they would fit anyway because the BBC is going to be larger than the ls1 in most dimensions.
1989 Mazda Rx7 GTU
Bolt On Ls1/T56 Powered
Status: Running... Most of The Time
11.68 @ 120 1.73 60' 3rd time out
Stock Ls1 ported TB W/ Full exhaust