Dreaded P0300 trouble code - again!

I have a 02 GMC Savana 3500 350 cid vortex. For the past year it has been throwing a P0300 code. Several garages have tried to track it down with no success. I eventually put a piece of black tape over the light. This week we put a new GM 350 crate motor replacement in the van. I also replaced the plugs, wires, cap, rotor, and distributor. I put the intake back on with new gaskets and fired it up. All went well and the engine fired right up with no leaks, strange noises or fault codes.(at first) After warming it up and checking everything out, we took it for a spin and all was great until the dreaded P0300 code came back. I cleared the code and refired it. At idle it does not throw a code, only at run speed. Also, there are no symptoms such as missing or stumbling. This is the same problem I had with the original motor. It runs and feels great except for the flashing trouble code. I am at a loss. I had hoped that this problem would have been solved with the new motor. Well I was wrong. I am thinking a new crank sensor or MAP sensor. I hate throwing expensive parts at it to try to solve this nagging problem. Any ideas would be greatly appreciated. Thanks Ron .:confused:

PS. I also had the injectors checked for flow with the old engine to rule out clogged spider legs in the injector system and all was good.

Usually installing a distributor sets a code, if it is off a hair, but no code for that....

I would put a scanner on it, and find out which cylinder(s) is misfiring.....and go from there.....

Update!
I put a new MAS and Crank sensors on with no change. One thing I noticed is that it only set the code when the van is at highway speed and u lift the throttle. In other words, when the engine is at speed and under the slightest load, there is no misfire. Only when u lift off of the gas does it set the code. I am going to install new O2 sensors and see what happens tomorrow. There has to be at least one mechanic that can tell me what the hell is wrong with this thing. I have spent hundreds of dollars having this problem in shops trying to track down the problem with no luck:crying:

PS. I have had a mechanic put the PC on it and the misfire is truly random - all cylinders.