Dear JNgent,
Among many alternative ways to find heat of dilution data, you may try and locate a book titled " Sulphur, Sulphur Di Oxide and Sulphuric Acid " written by UHF Sander, H Fischer, U Rothe and R Kola. This was published by British Sulphur Corporation in 1984 and had ISBN 0-902777-64-5 as reference.
I have the book and from this I can tell you that the heat liberated will be approx 750 kJ/kg of 100% H2SO4.
Good luck!