Hoods, tailgates, grills and dashes are different like Zenish said. Both have wraparound windshields 60-63, flat for 64-66. Chevy used two different dashes; 60-63 and 64-66. Both used torsion bar front, coil spring rear for 60-62. For 63-66, GMCs have leaf spring rears and coil spring front suspensions, Chevy used coil/coil unless we're talking 4x4s (leaf/leaf). Chevy gave a choice of inline 6 or small block v-8 in all years. GMC for 60-63 offered any engine you wanted as long as it was a 305 V-6. For 64-66 there was also a choice of an inline 6 Chevrolet engine. The 351 V-6 was an option for GMC in 1966. How's that for confusing?!
Here's a great site for 60-66 GMCs, there's a page showing the differences between the years.
http://www.6066gmctrucks.org/
Bob