Basically, the LD4B is designed for smaller, milder LA engines -273-318 - and has smaller ports while the LD340 is for bigger and more modified LA engines - 340-360. But there is no reason why someone can't use either one if whatever size LA being used has the appropriate mods for the particular application. (i.e.: LD4B on a 340/360 for a mild street build and/or fuel mileage for everyday driving or LD340 on a 273/318 with matching larger ports and higher rpm use.) You just gotta keep the overall combo right so everything works together.
The ones in the photo are both modified for use with a Thermoquad. They originally came with a square bore carb pad.
