If you are going to use a mnt crankset on a 520 you will have to replace the bottom bracket with a shorter one to pull pull the chain line to 45-46 mm.
I replaced the cranks on my Bianchi Volpe with a 44/32/22 crankset. The drive train is: Sugino DX500 crank (square taper), 103 mm botttom bracket, 11-34 cassette, LX rear derailleur, Tiagra (9 spd.) 4503 front derailleur, Tiagra shifters. I have this or a similar setup on 5 bikes in our family, and it works great. The DX 500 cranks are no longer available. You can use a Sugino XD 600 which can give you a 46/36/24. It will work with 8 or 9 spd., and your present bottom bracket will probably work.
