In the end I've decided for Amd K6 500 (6*83), with a voodoo 3 2000 pci and ESS 1 ESS1969F.
Actually something weird happens, when I use the 128x2 50ns memory the other 32x2mb won't get detected and pc goes nuts, while if I pair the 256 with another 32x2 they get correctly detected, I suspect the 32x2 memory modules don't work fine.
Initially I thought those were the fastest stable settings:
https://imgur.com/a/R3odoNE
Benchmarks:
Speedsys: https://imgur.com/a/F7OCNGB
Doom: 2134 gametics in 680 realtics
Quake 320x200: 70,9
Quake 640x480: 19,7
I was disappointed with the results of memory bandwith and transfer rate, it's clear enough that memory is what make the system crippled despite using 50ns and 83 mhz as bus frequency, I was considering to upgrade the cpu to k6-2+ but it's clearly not worth it.
Then I found that DRAM Turbo Read Leadoff and DRAM Speculative Leadoff when disabled let me use faster timings:
https://imgur.com/a/cxG1AJi
Benchmarks:
Speedsys: https://imgur.com/a/5N932K8
Doom: 2134 gametics in 677 realtics
Quake 320x200: 74,4
Quake 640x480: 20,4
As always any suggestion will be greatly appreciated.