efi-diy,
how much does a ported 4.2 cylinder head flow?
Most good aftermarket SBC heads flow over 300 CFM nowadays.
And 15% heads flow in the 360 CFM range,ported of course.
The Corvette LS6 heads flows like 360-380 CFM range stock,but they are CNC ported from the factory.A lot of R&D.
My 12 port head flows close to 320 CFM, but I am not done with it yet, & I have just the little above the street porting.
MBHD
