Excellent info, thanks! That explains why they sound like they do. Just not sure if the frame rails or accessories on a FJ40 will allow them to fit. There are dozens of off the shelf versions on Jegs. Was hoping somebody had already tried it.
Short answer: in the 20 years I've been playing with Cruisers, including doing a V8 swap on my first FJ40, I've not seen anyone do 180 degree headers. IMHO the benefit is just not there or what little benefit you might get would not be noticed. You are more likely to notice the increase of available torque off road with the use of a doubler or crawlbox than you will see with 180 degree headers.
I'd save the 180 degree headers for a max effort race car vs a 40 year old box on wheels.