They should be under the digital downloads section on Backerkit. If you asked for DRM-free keys, you should have a combined GOG key for both original games.
Aha! I found an email from them sent last year April saying "Today we’ll be sending out digital keys for Ultima Underworld I & II to Backers who have pledged and paid for the Adventurer ($75) tier or higher". I see you were quite generous :) +1 for that, and for taking the time to explain