What you want is 72.6*c*c = 6.5249626 × 1018 m2 / s2, or that many joules. Given 4.18e15 joules per megaton, we are left with 6.5249626e18 / 4.18e15 = 1561 megatons for the energy equivalent of 160 pounds.
As a check on the above, see http://www.1728.org/einstein.htm. Plug in 160, select pounds, and it computes 1559 megatons.
Your calculation implied a much higher mass to energy for a one megaton bomb than I remembered quoted, so I checked. The actual mass converted in a typical warhead is fairly trivial.
Not sure I completely understand what you're saying, at least not yet, but you seem to be saying that a 160 lb person yields the equivalent energy of 1,559 megatons of TNT. Is this correct?
I had initially, and mistakenly, used the units km instead of meters and came up with ONE megaton (1 million tons) of TNT. Then, when informed I was off by a factor of a million, by using km instead of meters for "c", the speed of light value in E=mc^2, reduced the final figure by a factor of a million to ONE ton of TNT. But now you're saying 1,559 MILLION tons of TNT??