460/429 question

my buddy bought a truck that the guy said it had a 429 in it. After finding the serial # on the block we checked it out and it came back as a 460. The seller said he used the intake, heads and crank from a 429 used with a 460 block to gain extra horsepower. Is this possible? What are the advantages of doing this and do you actually gain horsepower? The truck is estimated at around 500 hp