Good to see the RX-6 getting an engine upgrade. It should rip when spring rolls around.
When I made my engine mounts in my GN powered RX-7 (FC) I used ATR headers which allowed me to retain the stock steering rack location and also fit a 3" downpipe pretty easily. I actually used a stock 2.5" ATR s/s downpipe for a GN and it fit with zero mods!