Sounds very odd. The Wallstreet should pick up the EDID from the display automatically and since it is a recognised display, should also show the correct compatible resolutions. Possibly a driver issue? Might be worth looking up the latest drivers for your ATI card and installing or rolling back the one from the Universal.iso. This site has all the Classic mac ATI drivers.
https://gona.mactar.hu/ATI_Mac/
I have some command trickery to force an XGA resolution via Open Firmware but not the other way round. If you want to experiment
AYOR:
Start up your Wallstreet with Cmd+Opt+O+F
at the prompt
nvedit
dev /
3 encode-int " display-family" property
Enter Ctrl+C to exit the NVRAM editor then type:
nvstore
setenv use-nvramrc? true
reset-all
3 encode-int forces a resolution of 1024x768.
2 encode-int gives you 1152x768. I have no other confirmed parameters but if you want to try higher or lower numbers be my guest. There is always the risk of damaging some component or other but most likely it will just give you a garbled screen. Zapping the pram should restore matters to where you are now.