The Burns Merge collectors are impressive but on a stock engine they might not make much difference, But every little bit helps!
If there is a Tri-Y header I always recommend that. Any engine under 8000 RPM will see gains with that type over a 4 into 1 header.
180 degree headers are even better but good luck getting them to fit!
I recommend a proper Y or at least an X pipe. The gains can be dramatic.
For this I would do dual 2.5" into a single 3" Y pipe
https://www.summitracing.com/parts/flo-y250300s
with a WLK-17769 muffler
https://www.summitracing.com/parts/wlk-17769
and the biggest glass-pack after as a resonator.
This tail pipe kit
Free Shipping - Flowmaster Tailpipe Kits with qualifying orders of $109. Shop Exhaust Pipes, Tailpipes at Summit Racing.
www.summitracing.com
That would be optimum for 300 to 400 hp.