One activity with varying Resource requirements

We have a situation with a client who uses one activity with varying resource requirements. For example, an activity could be installing a concrete post, but the resource requirements vary depending up on the length of the post. 100 feet post may require 15 hours while 10 feet post may require 2 hours.  We can’t create multiple activities as the pole length may vary (could be 10 or 15 or 20 or 50 or any increment). We have hundreds of activity like this with different UOM (Example: Pour Concrete --.in Increments of 2 Cubic yards)  The resource are defined per UOM. Example : Hours per feet