The LD4B is pretty much a copy of a stock 340 intake, with a few improvements. Yes the runner size is a little smaller at the intake face to the head. That can be easily opened up if you desire. It also allows the easy use of the aftermarket carbs without the spacer/adapter that may be required on a stock 340 intake.
The LD340 has a much larger plenum, different runner design, and larger 340 size intake face to the head openings. It can also be easily modified to accept a Thermoquad without an adapter.
The real difference in hp or torque depends on your combination of parts and how you will be driving the car. Otherwise there is not as much of a power increase between the two with a driver.
Both are really good intake manifolds. The LD340 can squeeze a little more performance out of a more aggressive engine combination.
Too many cars, too many parts, too little coin, too little space to work in, too little time left to make it all happen!
Update: down to one ride, still too many parts, a little more jingle in the pocket, gaining space, and it's going to happen this year!
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Thanks A REAL MOPAR MAN ! 
