The Heidenau K37 is a great knobby tire especially suited for the Ural and a fan favorite. Using a progressive knob design, loading with mud or snow should not be a problem.
The 4.00 -19 M/C 71P TT is originally intended for sidecar use.
Heidenau uses a resilient compound, making for a tough knobby that doesn’t break, and last throughout the miles.