You are going to get heat regardless of whether you use a DC-DC convertor or a linear regulator. Energy needs to be dissipated when stepping down voltage. Unfortunately, silicon is not good at dissipating heat. That's why it gets hot. There is not enough surface area on the die to dissipate the heat. This is why the die is bonded to a heat sink in the packaging. This heat sink is usually connected to ground to prevent eddy currents. The heat sink of the package, depending on the energy that needs to be dissipated is connected to an external heat sink that further increases the surface area to help distribute the energy. This causes the regulator to run cooler because your heat dissipation efficency goes up.

Forget about using a DC-DC convertor. These can be a little complicated to setup for good line regulation. You could try a LM1084 which is an adjustable regulator that can supply up to 5A continuous with a maximum input of 8.8A. The maximum input voltage is set by the difference between the input and output. You are well within the 29V max difference. You would need a couple capacitors and resistors to set the cct up on a piece of perf board.

http://www.national.com/ds/LM/LM1084.pdf