There are no 486 class
bridgeboards or even Pentium ones. Period.
The best you can get are 386SX class boards (16 bit bus). Many can be upgraded with a
486SLC but that's far below a 386DX or even a 486. It's got a 486 instruction set (no FPU), a small cache and is faster than the 386SX but that's dog slow anyway.
If you abandon the 'bridge' concept you can use most CPU boards intended for passive backplanes - you'll have a completely separated PC running inside your Amiga and have to provide the communication yourself (NICs, serial/parallel, whatever).
I've run a 100 MHz Pentium board in my 3000 for a while but it failed to make a point with an Athlon Thunderbird sitting right next to it.