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With eight speakers in play I would be tempted to try two Stinger amps if they would fit. Since he has an aftermarket head unit and no CB module that should free up some space.
He might just find his speakers sound better with the increased quality wattage being fed to them.
I have thought about a Stinger spx350 on top of my SPX700.
If your happy withe the mono wiring strategy, then I'd suggest a Massive Audio TX48, plenty of power for those speakers, and the MMats 601CX in the fairing.