Without knowing why the clients said 72dpi we can only give you 'general advice'.. I'm also going to assume this is photoshop because you didn't mention a program.
In general terms you scaled it wrong.... all you did was adjust the dpi but forgot to change the dimensions to the original 59x39 inches, your 300dpi is literally no different to the 72dpi in terms of the total pixels being used.
What you needed to do was set the page size AND the dpi... you would need a page set at 59x39in at 300dpi.
Scaling your smaller page size up 6 times will make it no different to the original 72dpi image, adjusting the dpi won't make any difference to the 'crispness' of the image either as it's still working from the smaller image, it may make it a 'little' smoother but generally it just makes it blocky unless it's a vector type image.