His name was Paul Frees.
Bernard Grant was the voice for Volonte in both FISTFUL OF DOLLARS and FOR A FEW DOLLARS MORE.He was also the voice for Aldo Sambrell in NAVAJO JOE.
Voice talent records do not show Paul Frees as doing the voice for Volonte in the US English version of BULLET FOR THE GENERAL. They do, however, show Grant as the voice actor. And it is obvious that it is the same voice as used in the Leone films.
See, the guy who did the voice for Volonte in Bullet For the General - at least the DVD I saw - was markedly different from GMV's voice with Leone. I don't know what the clown on the SWWB is talking about.
Around 1968, when the film was picked up for American distribution, the distributors (like myself) found the International English dubbing to be very poor, very blandly delievered, and completely unexciting. So, they called for a new English language dubbing job and they hired the same company that had done the English language versions of the Dollars films. In this second English language version (which is what I am guessing you are calling the "new" dub, Sebastian)...the great thing is that Volonte is dubbed by the same voice actor, Bernard Grant, that did his voice in the Dollars films (no, Volonte's own voice was never used in the English language versions of Leone's films..regardless of what stories you might have read, or heard).
Is this a reputable source?
See http://www.absoluteastronomy.com/topics/Paul_FreesPaul Frees dubbed many Italian actors in Spaghetti Westerns, most notably Gian Maria Volonte in A Fistful of Dollars and For a Few Dollars More and actor Aldo Giuffrè the alcoholic Union Army captain in the Sergio Leone film The Good, the Bad and the Ugly in 1966....and Gabriele Ferzetti's Mr. Morton character in Once Upon a Time in the West .Frees also voiced Boris Badenoff, Ludwig von Drake, and the Pillsbury doughboy.