JohnRob:
The goal to enhancing thermal transfer is to fill the surface voids in both sides while having the thinnest intermediate layer possible.Without knowing more about the actual mechanical configuration, I would suggest you consider simple (or high performance) thermal paste and mechanically hold the resistors to the metal plate. I think this configuration would result in better thermal conduction because the thermal compound would (likely) be thinner and be optimized for thermal conduction without having to be an adhesive as well.
I agree, we need to know more about your project construction.
Using an adhesive of any sort will only compromise the heat transfer.
Thin is good..and its worth spending the extra.
Tom... 