Thanks for the replies. Very interesting subject.
The book I'm reading is "Shockwave: Countdown to Hiroshima" by Stephen Walker.
The book also states
"The day prior to Hiroshima four overloaded B29's that failed to get airborne and burned with their crews had been bulldozed to the side of the runway"
So I guess the problem of overloading the B29 was not just limited to Enola Gay.
I can't comment on the accuracy of the amount they were overloaded except from memory from reading the book Enola Gay was 7 tons over weight.