*Download Proxmox (https://proxmox.com/en/downloads/category/iso-images-pve) and burn to DVD/place on flash drive
*Install Proxmox on the host using CSM or EFI boto method (doesn't matter which)
*Ensure that the primary GPU in use by the Proxmox OS *is not* intended to be passed through to the VM (i.e. don't use the FX 5500 to run Proxmox)
*Don't install the Proxmox OS on a flash drive/SD card/etc (logs will kill the drive in no time)

*Add 'deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription' repo to file '/etc/apt/sources.list'
*Run command 'apt update' to... update apt!
*Run command 'apt dist-upgrade' to... upgrade the distro!
*Run command 'apt install xfce4 chromium lightdm' to "convert Proxmox into a workstation" (i.e. xfce4 GUI + Chromium browser)
*Run command 'adduser insertusernamehere' to create a lower-privileged user account for GUI access
*Optionally run command 'apt install sudo' to install SUDO (will also require editing /etc/
*Optionally run command 'apt install bolt' to install support for Thunderbolt adapters (i.e. Gigabyte Alpine/Titan Ridge, etc)
*Optionally run command 'apt install bluez' to install support for Bluetooth adapters (keyboards, mice, audio, etc)
*Add the below lines to file '/etc/modules' (enables VFIO modules required for PCI passthrough to work smoothly):

1vfio
2vfio_iommu_type1
3vfio_pci
4vfio_virqfd

*Create file '/etc/modprobe.d/iommu_unsafe_interrupts.conf' with the below lines (configures VFIO to be less strict about interrupts):

1options vfio_iommu_type1 allow_unsafe_interrupts=1

*Create file '/etc/modprobe.d/kvm_ignore_msrs.conf' with the below lines (configures KVM to ignore MSRs):

1options kvm ignore_msrs=1

*Create file '/etc/modprobe.d/load-vfio-before-xxxx.conf' with the below lines (configures Kernel to load VFIO module *before* the GPU modules):

1softdep radeon pre: vfio-pci
2softdep amdgpu pre: vfio-pci
3softdep nvidia pre: vfio-pci
4softdep nouveau pre: vfio-pci

*Create file '/etc/modprobe.d/vfio.conf' with the below lines (configures VFIO module to 'hook' the listed PCI devices at boot time to allow easier passthrough - in this case, 10de:0326 is the PCI ID for the FX 5500):

1options vfio-pci ids=10de:0326

NOTE: Do not list your Proxmox OS' primary video card in 'vfio.conf' - it should only contain PCI IDs for intended for PCI passthrough!
*Commit the changes to '/etc/modprobe.d' by running command 'update-initramfs -u -k all'
*Edit file '/etc/default/grub' with the below (enables/optimizes Intel's IOMMU and it's grouping, in addition to disabling PCI-E power management):

1GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on iommu=pt pcie_acs_override=downstream,multifunction pcie_aspm=off"

*Commit the changes to '/etc/default/grub' by running command 'update-grub'

1load-module module-native-protocol-unix auth-anonymous=1 socket=/tmp/pulse-socket

1apt install alsa-utils
2apt install alsamixergui

1alsactl init

1pcm.!default {
2        type hw
3        card 0
4}
5
6ctl.!default {
7        type hw
8        card 0
9}

*Argument '-machine hpet=off' is required to ensure VM performance remains consistent in 16-bit and 32-bit OSes
*Argument '-device adlib -device sb16' enables Adlib and Sound Blaster 16 devices in VM
*Argument '-audiodev id=audio0,driver=xxxx' tells QEMU/KVM where to route the VM's audio to (ALSA or PulseAudio)
*Argument '-object input-linux,id=kbd1,evdev=/dev/input/eventX' binds the VM's virtualized PS/2 keyboard port to a Linux EVDEV keyboard input device (found in /dev/input and /dev/input/by-path - make note of the symlinks in these directories!)
*Argument '-object input-linux,id=mouse1,evdev=/dev/input/eventX' binds the VM's virtualized PS/2 mouse port to a Linux EVDEV mouse input device (found in /dev/input and /dev/input/by-path - make note of the symlinks in these directories!)
*Parameter 'hostpci0' can use value 'romfile=gpuromfilegoeshere.rom' to define a dumped GPU ROM file located in '/usr/share/kvm/gpuromfilegoeshere.rom' (this is sometimes required for finicky Proxmox hosts)
*Parameter 'hostpci0' will will need value 'x-vga=1' if you are passing a legacy GPU through to the VM (FX 5500) to ensure the VM treats it as 'Primary'
*Parameter 'machine' needs value 'pc-i440fx-2.11' to ensure Proxmox loads the correct Machine Type (2.11 was the last version to include working legacy audio)

*Argument 'kvm' can have value '0' (hardware virtualization disabled) or '1' (hardware virtualization enabled)
NOTE: This setting does not actually affect PCI passthrough (works either way), but it *does* cause the HAL in the VM (Chipset/CPU/etc) to become emulated (instead of virtualized)... this is good from a compatibility standpoint, but bad from a performance standpoint. Toggle this setting if DOS/Windows 98 timing is too fast or too slow.
*Argument 'cpu' should have value 'qemu32' (artificially limits the CPU features in the VM to closely replicate a 32-bit only CPU) but optionally can instead have value 'host' to fully expose the Proxmox OS' CPU to the VM OS.

My go-to drivers for DOS are:
*JEMMEX
*HIMEM
*CTMOUSE
*MOSLO
*SHSUCD
*UNIVBE (works with virtualized Cirrus VGA!)

The only drivers needed in Windows 98 would be:
*Audio Driver (Adlib/SB16 drivers are already built-in to Windows 98 and use default ports)
*Audio Driver (Intel AC'97, attached as file "intel_ac97_windows98.zip"- hard to find!)
*Video Driver (Fx 5500, I use 81.98 from here: https://www.vogonsdrivers.com/getfile.php?fil … 952&menustate=0)

Nothing special here - upload a Windows 98 Setup ISO file to the Proxmox OS host, mount as CD-ROM in VM and install Windows 98 (or DOS) as normal.