I remember reading that CPUs with a 40MHz FSB have problems with VLB mobos. What can you tell me?
The early 486 was only available in 25 and 33 MHz versions. So early motherboards would have chipsets designed only up to 33 MHz. Likewise, early VLB cards would only have been designed/tested up to 33 MHz.
When the 486-50 came along, it became apparent just how much more difficult it was to get the whole motherboard, chipset and all VLB cards to work at 50 MHz.
This is what triggered the DX2-66.
There are motherboards/chipsets and VLB cards that can handle 40 or even 50 MHz, but it was not a standard speed, so you have to carefully select your components. It's basically a form of overclocking.
I personally own a 486 VLB system that is technically a 486DX2-66, but I run it overclocked as a DX2-80, so my chipset and VLB cards (Diamond SpeedStar PRO and a WinBond multi IO card) run at 40 MHz, zero waitstate, and it's been working like a charm for over 2 decades. So yes, it certainly is possible. But there's no guarantees.
Having said that, I use a real Intel DX2-66. I had one of those IBM Blue Lightning 486DX2-80 chips at first, and it didn't work properly on my motherboard. Not because of the 40 MHz FSB, but because it's a slightly different architecture, and my board/BIOS was not properly prepared for this.
I also have a Pentium OverDrive 83 MHz, which also does not work on the motherboard. Again, not because of the CPU, but because the board/BIOS isn't properly prepared for it.
Both chips work fine on other, more modern motherboards.
So you have to be careful, not all '486' CPUs work on all motherboards, and not all motherboards will support all voltages, speed grades etc. And not all VLB cards will be happy to operate over 33 MHz.
Late motherboards are a safer bet, then again, late 486 boards tend to be PCI.