please help if you can

[a men a truck a dog]

Ok so I bought a truck last summer it was a 1986 but a tree had fallen on the cab so the owner replaced the cab with one from a 1981 no big deal right ? The vin is on the cab so to get it registered I used the number in the cab no problem. Before I could drive this truck I had to replace the motor and trans so these three things I'm clear on motor and trans vins are clean and the cab vin is clean because I got a tag under it I was pulled over by the cops when looking at the truck they first thought was that it was stolen I just got it running good haven't painted it or anything so even doe the hold truck is black you can tell things not the same easy. Now they have been holding my truck for over a week trying to find out something is wrong with it anyone got any advise as to what I should do
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