it may take a while to get through to the seller. I think you could use the F5 boards I sold you, but try and get the devices as far as possible apart, or look into using heat spreaders to make sure you are using the depth of the heat sink to its maximal potential. Also, consider looking at the more exotic isolator materials, as I'm finding these to be an excellent performance increase. As long as you're not absolutely set on hitting the bias used in the First Watt, I think you'll be okay. If you actually going to do the math instead of a simple "hand test" after the fact, remember you have to derate the Conrad heat sinks as the C/W is quoted at a much higher ambient temperature than you want to run your devices at (80 C). One you start to really look at the numbers, (and appropriate implications due to the fact that powder coating doesn't perform as well as anodizing) these IMHO are not optimal for use in a power amplifier. But given the caveats above, you should be okay.
edit: The R-Theta GB (which is ending soon, as is under the title Aavid Thermalloy), IMHO would be a better choice for a F5.