How? They don't have to build it, just bore it and cam bearings. The processes are the same. just the buick bore is smaller by .2. And buicks all they have to do is make sure the rods and mains are no bigger than .002. If they can't do that well then... you know. If you got that one friend willing, take him up on it. Every thing short of boring and cam bearings you can do yourself.
They won't warrant the engine if they don't assemble it. Sure, I could assemble it myself, but what if it throws a rod shortly after like yours did? Then I'm out my money, and I'm in no better of a spot than I am now.
Even if I do assemble it myself, it will take me a lot longer than it would to just buy a motor and drop it in. I want to have this thing up and running sooner than later. The one machine shop has a 350 Chevy 4 bolt truck motor that's .030 and has a mild cam in it, and is complete carb to pan, and he wants $1800 for it. Belonged to a customer who never paid for it. I think at this point, I'd buy that before I put money into the Buick engine.