It is probably a bad idea to seperate the PA transistors (IRF510) too far from the circuit board. Since they are trying to work together in a push-pull arrangement to drive a few amps of RF current through a transformer at high frequencies you are going to run in to all sorts of problems that will not be easy to predict or replicate for each different installation.
Determining if the heat sink is sufficient requires quite a few data-points;
1. Power level you will be running at
2. Band you will be operating on
3. SWR that the amplifier sees from the antenna system
4. DC supply voltage and current
5. Duty cycle of the transmitter and mode of operation
6. Type of heat sink (aluminum finned, copper, just a steel plate)
7. Acceptable temperature rise of the PA
8. Thermal conductivity of the junction between the PA and heat sink
9. Convection cooled heat sink or forced air
You can see why many designers will put some margin in heat sink selection or incorporate devices to measure temperature at the PA to fold back the power or put the amplifier in to protection mode to prevent the magic smoke packets from being released.
Tisha Hayes, AA4HA
Sr Engineer, 4RF Inc.