Engine origin

[Dcm]

I have a 1970 swb half ton fleet side with cool spring rear suspension. The info from decoding the vin says the truck came from St. Louis and had a V8 and a 4 speed manual tranny. The engine "looks" original (ram horn exhaust manifold, chevy intake and heads, quadrajet carb). But when I ran the numbers on the engine it came up with an age range bracket of 74-78. The numbers on the intake point to a car from mid 70's with a four barrel. The vin on engine doesn't match vin on truck. Is this a fairly common occurance and are there any "gotcha's" to watch out for?