Flow numbers for new vortec heads sold by Pace. The stock Vortec 305 heads flow at least as good. If your stock 99 ones check out good use them, otherwise go aftermarket Vortec.
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This is what my out of the box Assault castings with zero work and untouched seats and aftermarket valves not yet even cut. A little bowl work and decent valve job gets these up around 265 cfm @ 0.600.
LIFT------INTAKE-----EXHAUST
.200----------135-------103
.300----------183-------131
.400----------225-------150
.500----------244-------164
.600----------241-------175
.700----------241-------177
After Lloyd Elliot did some work to them.
LIFT------INTAKE-----EXHAUST
.200----------138-------107
.300----------192-------138
.400----------241-------164
.500----------274-------183
.600----------295-------193
.700----------299-------199