I want to connect old Quantum hard drive( 500MB FRBLS 655-0272 SCSI 50-Pin)

I assume that you want to access files stored on the older drive. Check eBay for an Adaptec Power Domain 2906 or 2930 PCI SCSI controller card for Macs (there are PC versions which won't work). Either of these cards has an internal 50-pin connection designed for the same 50-conductor ribbon cable used with that hard drive. Mount the drive internally and connect one of the unused molex power connectors to it. If you just want to connect the drive occasionally, you'd want to install it in an external SCSI housing that would typically have a pair of Centronics-50 ports on the rear. The Adaptec 2906 has an external DB-25 port, so a standard Centronics-50 to DB-25 SCSI cable would be needed. The Adaptec 2930 has an external HD-50 port on the card, so you'd need an adapter or different cable to connect it to the external case.