This is the system i installed from Ram Air Restorations

UESYS-2C Ultimate System for FB Convertible

Ultimate exhaust for 67-69 Firebird Convertible with our X-crossover which fits above the X-brace. No need to lower the brace and give up valuable strength & ground clearance. Includes downpipes for Long Branch factory headers and ball&cone connections that install in the front of each muffler so the system can easily be removed and reinstalled. Mandrel bent tails go all the way to the rear. If you want tails that turn-down behind rear tire state your preference in order comments. All American made Lifetime warranty. Comes with easy to use rod hangers, clamps and instructions. Welding is recommended.
Mufflers that fit in this system are listed below. Best fit are the 3334F Spin Tech as it is only 3" tall which maximizes ground clearance. Other muffler options are 4" tall.

I also paired it with the Long Branch Manifolds for my 400 HO

Thoughts. Could have used some instructions for installation. Though I was able to contact tech support and they could provide answers. Even though this kit was made specific for my vehicle it required some pipe trimming. This was my only complaint, i mean if its built for my car why shouldn't the pipes fit exact, plus small turns in the orientation changed the length of the pipes, none of the pipes are labled like "a" goes to "a" etc. Long story short, installed myself and the car sounds amazing. Also I am spoiled and have a lift in my garage, installing on the ground would not be any fun and I did have to do some welding. I am in the process of replacing the transmission in my car so I can report back on how easy it is to move it out of the way, it came with clamps to make the exhaust removable, I will be putting that to the test next. Overall besides installation could have been easier the exhaust is amazing, but expensive!