Slightly lighter 1964 body, not by much.
Engines would have been the same or slightly heavier in 1964. The Tri-Power manifold in 1964 weighed a whopping 46.2 pounds bare:
https://forums.maxperformanceinc.com...5&postcount=45
The 1965 Tri-Power intake was a lightweight at 34.8 pounds:
https://forums.maxperformanceinc.com...0&postcount=52
I guess if there was a significant difference in block casting weights who knows but essentially the same.
1965 #77 heads were an improvement over the 1964 9770716 heads, horsepower ratings for the standard 4-barrel 389 went up from 325 to 335 using the same cam (524009 in 1964 was the same profile as the 9779067 in 1965). The Tri-Power going up from 348 to 360 isn’t a good comparison because the 1965 used the bigger 9779068 cam.
1964 fan here, fun conversation!