The LT4 intake (12550630 or 12550631) has identical runners to the late model LT1 intake (12552137 or 12552138)
but has more material above the runners so that it will seat properly with the LT4 heads. The extra material
over the port is to seal the smaller LT1 intake ports to the larger LT4 head ports. Additionally, LT4 intakes were
powder coated RED from the factory. These are the only significant differences between the two intakes.
Note, however, that the LT4 manifold will NOT fit LT1 heads. They will bolt up, but the ports will not match
and there will be interference with the valve cover gasket rail on the LT1 head.
See the following photos to note the physical differences (these intakes have been bead blasted).
And just FYI, Edelbrock now makes an aftermarket LT4 intake manifold...
As per Edelbrock's marketing material: "Utililizing the proven short-runner tunnel-ram design for 1996 LT4
engines, this manifold incorporates a .250" longer-than-stock runner and Edelbrock�s patented Air-Gap� design
that delivers a cooler, denser air stream for maximum power from 1500 to 6500 rpm. The throttle bore inlets are
cast larger than stock to accommodate a 52mm or 58mm throttle body. Early and late OEM LT1 throttle bodies
and fuel rails may be used with this manifold."