There were many interesting things in 67 and 68.....Are You Experienced came out before Disraeli Gears and imho Purple Haze is 'heavier' than Sunshine Of Ypour Love. The KInks track is cool because Dave Davies had a broken amp that gave it that raw sound but in the end the song is not that heavy.
Many in the heavy rock metal area think that Vincebus Eruptum by Blue Cheer was possibly one of the first lps to haver that overamped thick sound and it was mostly blues tracks also. They did the first loud raucous version of Summertime Blues.
If this aint early heavy metal...not sure what is ......
Vincebus Eruptum and Outside Inside both came out in 1968 and imho are heavier than anything else at that time....only Cream and Hendrix are close....but they both were more polished so Blue Cheer sounds harder edged.