If the cpu is removable on the accelerator card, then why not just pop it into the quadra and swap the onboard oscillator. It won't be as fast but at least it's faster than what you got...
"Quadra 610
The 25 MHz Quadra 610 replaced the 20 MHz Centris 610 and uses the full 68040 CPU, not the FPU-free 68LC040 version. The oscillator runs at 12.5 MHz and the CPU at twice that internally, 25 MHz. Using a 14.7456 to 15 MHz oscillator will boost speed to 29.5 to 30 MHz while allowing the serial ports to continue to function properly.
If you don’t need to use the Mac’s serial ports, you can over clock the 610 to 37 MHz with an 18.5 MHz oscillator. You can drive a Quadra 610 with DOS card to 34.9 MHz using a 16.96 MHz oscillator."
- From Low end mac
"Quadra 610
The 25 MHz Quadra 610 replaced the 20 MHz Centris 610 and uses the full 68040 CPU, not the FPU-free 68LC040 version. The oscillator runs at 12.5 MHz and the CPU at twice that internally, 25 MHz. Using a 14.7456 to 15 MHz oscillator will boost speed to 29.5 to 30 MHz while allowing the serial ports to continue to function properly.
If you don’t need to use the Mac’s serial ports, you can over clock the 610 to 37 MHz with an 18.5 MHz oscillator. You can drive a Quadra 610 with DOS card to 34.9 MHz using a 16.96 MHz oscillator."
- From Low end mac