Thats exactly what I did John, had a 0-50,0-50 10A toroidal, wound 10 turns of wire round it (keeping them spaced evenly), measured the volts given then adjusted the turns to provide a nice 8v DC for the 5v regulator.

Note: If you are using John's idea in a 'buck' (reduction) arrangement you should measure the output volts when on load to avoid it not dropping as much or more than expected. Else assume a 5% regulation. So for a 70v DC output you need a 50v AC, if you have a 55vAC toroid it may drop to 52.5vAC on load so if you buck 5v you may only have 47.5v AC (67v DC) on full load. You may only want to buck 3.5v - depends on what compromise you decide.