I've successfully installed two A3640 cards (the processor card from the A4000) into two different A3000 computers. Yes, it's possible. Two 'must have' requirements are that the A3640 board needs to be revision 3.1 or 3.2, and the A3000 needs Kickstart 3.1 ROMS. Success also depends on the revisions of the DMAC, BUSTER, and RAMSEY chips, but this is only something to pursue if you're unsuccessful in your fist try. Other than that, you'll need to figure out the proper jumper positions for the A3000 motherboard (do a search, I've run across multiple sites with this info), and make sure you have a low profile heatsink.
The INT2 hack is only required for accelerator boards which have SCSI or a PPC processor also installed, which the A3640 does *not* have.
(FYI, if you every try to upgrade the A3000 with your CyberStorm 060 board, a known issue is that some earlier revisions did not like fast memory installed on the A3000 motherboard, it may need to be removed. Also, the jumper positions are different than those required by the A3640 board.)
:-D