honestly I'd pick up another t-63. They're rated for 750hp worth of air at 30psi which means about a third of that plus a bit at 15psi per head unit. If you're looking to get about 600hp from it (a good number for a turbo 5.4) at moderate boost, then twin t63's would be just about right.
If you're worried about turbo lag then drop down to a t-61. You'll have boost instantly with the 61 and just a hair of spool up time on the 63.
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