Well for those who want to convert a single plane SBC manifold over, you have 3-4 major problems that you have to overcome. First the bolt angles are different the SBC bolts are perpendicular to the mating surface. The LT1 bolt holes go straight down. Also the spacing is different. Another problem is that the valve cover sealing surface for LT1 come up away from the flat surface that the intake manifold seals against. So the outer edges of the SBC intake manifold are fatter and hit this.
This first picture shows the LT1 head and an LT1 EFI intake in place, notice the lip on the head just outside of the manifold. If you look at the other pictures that show all the manifolds together you can see the extra material on the SBC manifold that interferes.
From left to right LT1 Carb, LT1 EFI (1995 Vette), SBC Carb
Look at the difference in material around the bolt holes.
The rear of the manifolds where the distributor goes.
The bottom of the SBC Carb intake, you can see how the bolt holes are perpendicular to the mating surface.
LT1 Stock EFI manifold, you can see how the bolt holes are more vertical than the SBC.
Conclusion, buy the GM Carb intake. Unless you spend a whole lot of time north of 6,000rpm, then pay someone to mod a Victor or make a sheet metal for you. I made 580 fwhp with the GM Carb intake on a mild cam hydraulic roller 396