My understanding (and someone correct me if I'm off a bit) is that the exhaust gas cools down significantly after the mufflers. As such the tail pipes do not need to be a large as the head pipes.
With your build I think the 2 1/4s will be fine.
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Ed
1968 GTO (Thanks Mom)
2006 Silverado
2007 Cadillac SRX
2015 Chevy Express 
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