Your options are to either use all the +12v (yellow cables) and all the Ground (Black cables in paralel) or simply cut them very short and solder them as a bunch on a much thicker cable to feed the heated bed.
However even if your ATX psu is puting out 58amps it may only do so for short periods of time, and only when the other rails have some load... 5v rail comes to mind.
I had a similar problem with my old ATX psu, it was suposed to put out ~22amps, which would have been enough but the voltage drooped on the 12v rail down to 10.5-11v even with load on the 5v rail via large wire wound resistor. At this point I decided to ditch the ATX psu, and finding the chinese standalone PSU's not trust worthy, I opted for a 12v only server psu manufactured by delta. It is not the quietest thing on the planet, however even with the heated bed on, and a 150w auxiliary heater on it never really droops below 12v. I used heavy gauge wire soldered into its terminals and have seen stable power out of it for the last few months. Voltage droop is caused by thin wires heating up, or even a weak PSU which is not exactly living up to standard.
My particular model is rated at 53.3A with 110v input and 106.5A at 220v..... Here is a link to the work necessary to get my particular model to work. PSU If your counting the psu is puting out 650w at 110v and 1300w at 220v. those are the continuous ratings for it. it can put out much more, the guys at RC forums are getting nearly 1500w out of these with 220 and nearly 800w with 110v. In many respects the server PSU was much easier to modify than the ATX psu I used before. I simply soldered cables to it, and used a 2 pin jumper cable wired to a switch to turn it on or off. The down side to this psu is that it does not have a 5v rail if you need it you need to run a converter. I used a cigarette lighter plug with usb ports on it to power the wifi module for my particular setup.
However even if your ATX psu is puting out 58amps it may only do so for short periods of time, and only when the other rails have some load... 5v rail comes to mind.
I had a similar problem with my old ATX psu, it was suposed to put out ~22amps, which would have been enough but the voltage drooped on the 12v rail down to 10.5-11v even with load on the 5v rail via large wire wound resistor. At this point I decided to ditch the ATX psu, and finding the chinese standalone PSU's not trust worthy, I opted for a 12v only server psu manufactured by delta. It is not the quietest thing on the planet, however even with the heated bed on, and a 150w auxiliary heater on it never really droops below 12v. I used heavy gauge wire soldered into its terminals and have seen stable power out of it for the last few months. Voltage droop is caused by thin wires heating up, or even a weak PSU which is not exactly living up to standard.
My particular model is rated at 53.3A with 110v input and 106.5A at 220v..... Here is a link to the work necessary to get my particular model to work. PSU If your counting the psu is puting out 650w at 110v and 1300w at 220v. those are the continuous ratings for it. it can put out much more, the guys at RC forums are getting nearly 1500w out of these with 220 and nearly 800w with 110v. In many respects the server PSU was much easier to modify than the ATX psu I used before. I simply soldered cables to it, and used a 2 pin jumper cable wired to a switch to turn it on or off. The down side to this psu is that it does not have a 5v rail if you need it you need to run a converter. I used a cigarette lighter plug with usb ports on it to power the wifi module for my particular setup.