Ok... Be patient with me, because I might sound harsh here, but that's not my intention.
Here's what this sounds like...
You bought the wrong part for your van, your mechanic installed it anyway, and you don't know why the van doesn't work properly?
See what I mean?
I'm not an expert, but I doubt that General Motors swapped the front and rear circuits on their master cylinders and made no other changes to the system.
Your "mechanic" should have had you return the incorrect part and get him the correct part before doing anything else.
I mean, it's your brakes... why chance it?
Your issue could be a bleeding problem I suppose, but honestly, the only thing I'm sure of is the incorrect master cylinder.
If it was mine, I'd get the right part, install it, and go from there.
Even if you still have the pedal travel issue afterwards, at least you'll know it's not because of part incompatibility.
The other thing is: Even if you get it working how you'd like it with the wrong part, it's kind of oddball now and is no longer built like it was from the factory.
So it no longer conforms to the service manuals, or the memory and knowledge of mechanics who may work on it in the future.
It's something you'll have to remember, and it's something a future owner will have to figure out.
I hope that all came out ok, and I didn't sound like too much of an ass.
However you go with it, good luck, and get it fixed so you can drive it.