Vin #'s, motor swaps, and PO's, now I'm confused

[1968CHEVYC10]

So I've had my '68 for over a year now, It belonged to my grandpa before that. He told me it originally had an inline 6 in it but was swapped sometime before 83 for a 350, according to the code on the passenger side near the water pump the engine is a '75, I'm pretty positive this is correct, also when I replaced the oil pan gasket only the 75+ one would fit So i think that confirms it. Now the vin # on the door jam says its a 68 with a v8, and the SPID also says the same thing, so I just wondering, why would someone change the vin? and I didn't know you could even change the SPID back then anyways, was/is it required that you do that? I live in TX and I'm pretty sure it's always been a Texas truck.