I have heard of it, but only in bigfoot circles, not through any other source. I think it's an urban legend that has been touted by the government conspiracy crowd for quite awhile.
He did sign the Federal Wilderness Act in September of 1964. No mention of bigfoot in the speech as far as I know.
QUOTE
If future generations are to remem-
ber us with gratitude rather than
contempt, we must leave them more
than the miracles of technology. We
must leave them a glimpse of the
world as it was in the beginning, not
just after we got through with it.
~President Lyndon B. Johnson
Redwolf