I also have a set of never installed knockoff headers. The only issue with them is the crossover pipe is about 1" off from fitting properly. I do have a flex coupling that I was intending to weld into the center of it so it will fit properly. Unfortunately other projects are higher on the to-do list than these. $100 plus shipping.
Gbodyparts says their 3” DP is designed after the ATR DP, but guys say they don’t fit the best????
The RJC 3” DP is rumored to fit with ATR headers, but would need to be confirmed by someone running it.
I had the Bowling Green Customs 3” wastegate elbow when I installed ATR headers in the early’90’s. I made a 3” mild steel DP to fit because BGC didn’t and went through this dilemma 5 yrs ago when I was looking for a SS DP. I switched back to my stock headers because the Gbodyparts 3.5” DP I got didn’t fit.
Gbodyparts had just switched over to selling the TA Performance 3.5” DP and their sales/ tech guys didn’t know it wouldn’t fit.
Cruz Performance is now making ATR/ KB style headers and should be able to make whatever DP you want if you don’t find one.