Some PLCC68 RAMDACs to try once I add the missing 2 MB DRAM.

I also have the ATT20C408 on order.

Check out this video - later version of your card with I'm pretty sure 4MB of memory: https://www.youtube.com/watch?v=4rxM2Z-CUCE
edit: nope I'm wrong, those are multiport VRAM chips so only 8 bits per chip, huh.

Looking at the pins for the data bus on the SOJ memory pads, there are resistor packs with two possible locations. I'm pretty sure that for example the RP15 / RP36 are hooking up data lines of the chip to the memory and RP36 which traces to the data pins for the SoJ is not hooked up. So the 100-ohm resistor packs will probably need to be added.
Then there's some 0-ohm resistor packs at RP9 / RP42 - from what I know of RAM on the Voodoo 1 those may be the RAS / CAS lines. See where the RAS & CAS lines on the existing RAM hook up to.

To aid in figuring that out, take a hi-res shot - completely perpendicular to the card so there's no skew - of the front & back of the card. I use a CCD scanner or take pictures with a zoom lens to flatten out the perspective that can cause distortion. Put them into an image editor and set the back-side to 50% opacity (or 'lighten' in the layer mode, I found that's good). Then you can trace out where the traces go to, to figure out what traces hooks up which signal.
The RAS, CAS, Write-Enable and Output Enable pins are probably grouped up on the QFP i.e. all CAS pins would be in the same spot in order. So if you find them for the existing RAM then the ones for the unpopulated parts should be nearby.

Another thing is, are you going in blind here? I was lucky and found a schematic that includes the Voodoo 1's complete pinout, but if the datasheet / pinout for your card aren't know, you'll need to reverse engineer a few traces on the card to figure out what needs to be populated. It's slow going but absolutely possible since the pinouts for all the memory chips is known and you can refer to how the traces are hooked up on the populated areas for what resistor values are needed.
Something I'm not sure of is what the resistors between the SoJ chips like R8 are for, or what the resistor spots at the top are for, perhaps those are pull-up / pull-down resistors.

Lastly, there's the bypass caps, make sure to include those 😀 The capacitor pads inbetween the SoJ chips would need 100nf capacitors typically.

I'm pretty sure the card in that video is for the VRAM variant of the Mach64 VLB. You could find those cards easily 20 years ago with the upgrade connector in place. My Mach64 uses regular DRAM; I haven't seen a DRAM VLB (or PCI?) card with 4 MB. Did they exist? I'm not sure why my VLB card has the solder pads for a 2 MB upgrade.

Thanks for your comments. I won't be starting this work for some time yet. I just wanted to get a thread started to see which comments come in between now and when I do get started. As I am currently fiddling with RAMDAC's on another card now, I will probably test different RAMDAC chips on the Mach64 to first see which are functional.

I haven't looked for a schematic yet. I would be surprised if one was public.