What type of monitor can live in a 70 gallon tank?

70 gallons means nothing - it's the physical dimensionsthat are more important. The lizard needs at least the length of its body (at adult size) for the longest side of the tank. You also have to take into consideration whether the monitor is an arboreal species (and would therefore need a high enclosure)

Incidentally - old fish-tanks do NOT make good vivaria for reptiles. Being of all glass construction - they do not hold the temperature generated by the heating system - and the thermostat (and heat source) have to 'work overtime' in order to maintain the desired range. 90% of heat generated will be lost through the glass - as opposed to around 5% in a purpose-built wooden vivarium.