You will not get efficient thermal transfer to the pins with any kind of solder. Especially ones containing
Lead or Tin. Even the highest silver content solder (=95%) has less than 1/4 the conductivity as pure copper.
Like Talcite said, if you have access to CNC see if you can machine the base & pins in one piece.
~ Thermal Conductivity ~
Copper = 398 W/m-C
Hard silver solder (95% Au) = 94 W/m-C
http://www.engineersedge.com/properties_of_metals.htm