With a stock computer, even recalibrated, your part throttle cruise is going t be 14.7 because the computer is designed to run part throttle at stoich off of a narrowband o2 gauge. The only other option would be to disable closed loop o2 monitoring during cruise but I somehow doubt your tuner will do that.

Stock mechanical advance distributors + vacuum advance could give you a total advance in the low 50's at a steady cruise. However the magnum headed engines will require less timing at cruise than the old open chambered engines of the 70's.