An American V8 has a 0-180-90-270 or 0-180-270-90 crank. If you use a 4-1 pipe with roughly equal header lengths on each side, you get the Detroit Rumble, with uneven spacing between exhaust valve opening "hits" on each side.
If you want it to sound like a Ferrari V8, you need to get the pulses to show up at the same time they do on a Ferrari V8.
A Ferrari V8 uses a 0-180-180-0 crank. You'll need to fake this. Fortunately, you have one cylinder firing every 90 degrees just like the Ferrari, so you need to link 'em that way.
An LS1 has a 18726543 firing order, with 1357 and 2468 on each bank. The exhaust pulse order you have right now is 1753 and 8264. To get what you want, you need to cross over some pipes so you're running 1764 down one pipe and 8253 down the other. You will need to get the primary lengths on each header to be the same.
Good luck.
If all else fails, build a set of very short equal length headers terminating in the hotside intake of a pair of turbochargers. Turbochevys are awesome.