YesLOL - keep in mind he also thought Yoko would be a great vocal addition to the band. Nope, not even a contender in the birth of Heavy Metal.
All my searching uncovered Iron Butterfly singing In A Gadda Divida in April of 1967 (possibly even earlier than that) with record releases in early 1968, very close to Black Sabbath launching their Metal tunes on their second album in 1968 also. It kinda comes down to wether or not you consider Gadda hard enough to be Metal (IMHO In 1968 it was viewed as a clear departure from just about anything else that was being played). At that same time in musical history, the Beatles had just released "All You Need is Love" (not exactly Heavy Metal, Metal or even a Hard Rock). I think the first ones to cross the line were IB followed very closely by BS and then by LZ and DP.
But any way you look at it, 1968 was a great year for music and a great year for muscle cars!
1968 Monaro GTS 327