Yeah, it's difficult, but there are some basic principles to keep in mind.
First is, your primary pipes and collector need to ideally empty into open air, or a large enclosed volume that is big enough to work like open air as far as exhaust tuning goes.
How big it needs to be depends on individual cylinder displacement, but bigger is better to prevent detuning the collector length.
The eight inch pipe across the car sounds like a very good idea, if you have a rear mounted engine.
An eight inch pipe along and under the car will work too, if you have sufficient ground clearance to do it.
This idea is not new, it is mentioned by Vizard, and Dr Morrison in his book "Scientific design of exhaust and intake systems", and by others. And it works.
This big stored gas volume does two things, it minimizes collector detuning, and it reduces low frequency exhaust pulsing, but only if this big "exhaust tank" has a restricted exit area to reduce noise.
How big the exit area needs to be depends on gas flow, which depends on power. Assume about 2.2 CFM per Hp.
You can just run a small bore exit pipe, and it will be quiet, even with no muffler, and it WILL NOT DRONE.
But that would cause far too much pressure drop at max power.
A larger bore exit pipe (or pipes) with glass packs or straight through mufflers will still be fairly quiet, because there will be much reduced low frequency pulsing, and only the harsh higher frequencies to deal with.
The very best results for a sleeper will be with mufflers fitted with exhaust flaps that close almost right up at low flow, and progressively open at higher power levels.
Note the spring loaded flaps fitted inside this muffler.
http://www.dynomax.com/images/mufflers/vt_lg.jpg
One, two, (or four!) Dynomax mufflers in parallel right at the back (depending on power level) would be ideal.
Building a quiet low back pressure exhaust is not that difficult, provided you have sufficient room, which is usually the biggest limitation.
For the turbo guys, just fit a very large pipe right to the back, and then the smallest flap type muffler that will work right at the very back. No drone, very low noise, and low back pressure flat out.
Just fitting a flap muffler right on the end of a header pipe will not work.
There ABSOLUTELY MUST be a big stored gas volume between the engine and flap type muffler to reduce low frequency pulsing for this type of muffler to work at all.
Some guys just stick a flap muffler on their car without doing anything else and say the muffler is crap.
It is still far too noisy. But if you build in sufficient exhaust volume before the muffler, they work like magic.
