First off, no the engine should not leak.
Second you keep saying you spent 12k(ish) on the engine. Add up the price in the parts alone: dart block(plus all the machining involved- which is a lot), crank, rods, pistons, rings. heads, intake, cam, pushrods, oil pan, oil pump plus all the associated hardware. It adds up fast for quality parts. You act like the builder made 12k on you. Not even a fraction of that.
With that said, it doesn't excuse the leak.
You came here looking for everyone to burn down the builders building. It seems it didn't work out that way.
You keep repeating yourself, and just about everyone is giving you other opinions, and you can not accept it.
Carry on. Let's see how long you are going to complain about the builder before you find where the leak is actually coming from. The builder is not going to be able to tell you where it's leaking over the phone. He definitely not going to pay someone to fix it in the car. That was never part of deal, like it or not.