1991 Sub 2500, VIN ID crisis

I wrote about my truck in an earlier post (91 SUB/ 2500 OBD1 codes 22,33) The latest is that I have apparantly 3 VIN codes on the vehicle. Only one matches what I bought. 2 of them are for 89's with a 454, and 1 is an
"assigned vin" that matches the 91/353 that I think I own. My local vehicular surgeon's have informed me that they can go no further. They eliminated everything. It runs fine when you accelerate but it wont idle. As soon as you take your foot off the gas it sputters and dies. They say it is getting way too much fuel at idle. They looked at the brain, it had a high performance chip in it. They said the brain is for the 89'/454 engine and blame the chip. The chip (hypertech)is for a 90'/350 and hypertech blames the truck. Both say to get the brain/engine/year to match and go from there. Well, which brain, which year? Does anybody know the best place to check the VIN, my dash doesn't have one on it. The glove box reads 1989, but it might not be the origional glove box. The "assigned VIN" is on the driver door. I cant affored to by 2 or 3 brains as a test. Any suggestions? And can anybody give me a clue as to why the truck ran at all with the mismatch?
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There are places on the frame that have the vin , how about the door plate? Take out the seats in the rear look for sheets from the assy line.

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