I'm curious as to which Carrier card you were testing with. Somewhere around here I posted undocumented speed settings for one of the carrier cards (XLR8?). It's been too long and I don't remember which one. They give some speed settings in the instructions that came with the card, but it turns out the thing is adjustable in .2 MHz increments from something like 40 MHz up to about 70 MHz, IIRC. Anyway, that might get you over 58 MHz for what it's worth.
In my experiments, I could never get a bus speed higher than about 62 MHz to operate. There was an old 604e upgrade card from either XLR8 or PowerLogic which came with its adjustment switches on a little board at the end of a ribbon cable, so you could run them out of the case and adjust the speed without having to open and close the case constantly. PowerForce Pro or something like that. Definitely had the Pro at the end, because there was a non Pro version that was the same card, but without the ribbon cable -- switches were on the card inside the case on the non-pro version. Interesting thing about that card was that the speed adjustments ran through a little PIC on the card which took the settings and translated them into inputs for the clock generator and also for adjustments to the CLKID pins.
I'm going to bet that you were using an XLR8 brand card because they built their upgrades to take advantage of the CLKID pins in the CPU slot. These three pins signal the logic board chips as to what bus speed will be used so that memory timings and such can be adjusted properly. Without adjusting the CLKID pins, PCI Power Macs generally won't run bus speeds above 50 MHz and it's tough to get much over 45 MHz.