I am looking for a thermally conductive compound that would most efficently transfer heat at the interface of a steel block with copper tube passing through it.
Here are two heat transfer compounds that we use for tracing purposes. One is from CSI and the other from Thermon. They are numerous others out there and if none of the the above information doesn't cover your requirements.
You might want to look at the CSI strap-on tracing It can be taken of and reused many times.