You have to account for it with the heads and block, not the intake. I'm sure there are plenty of people in the LSX world that have messed with their engines but still retain a plastic intake of one type or another.
On a big block, I think a full plastic intake is probably pretty easy. For a small block, maybe you make an aluminum lower to mate to the heads and have the water crossover and plastic uppers. Then you could have swaps between different length runner intakes, etc, etc with the same lower.
I do remember seeing a SBC AFR composite intake some years back but I don't know anything more about them.
You would think plastic would do a lot better with being able to make a thinner wall mold (and therefore big runners if needed) and also not conducting heat into the air fuel mix. Also a better surface finish. Should also be lighter.
