There are some 3 and 5 amp regulator chips in TO-3 cases
which should handle your power level just fine. Another
option would be to use the LM317 to drive a pass transistor, such
as a 2N3055, which would handle most of the current. Look in the
ARRL handbook or similar reference for a circuit with the transistor
"wrapped around" the regulator: this allows some of the protective
circuitry of the regulator to keep working.
Make sure you use a tiny bit of heat sink compound between the
regulator and the heat sink: just enough to fill any gaps.
You can also reduce the heat dissipated in the regulator by
lowering the input voltage. If the rig is drawing 1 amp at 13 volts,
and you are feeding 20 volts to the regulator, it will dissipate
(20 volts - 13 volts) * 1 amp = 7 watts. Reducing the input to
16 volts will reduce the regulator power to 3 watts. (You have to
maintain at least 2.5 volts difference between input and desired
output with most regulator chips, though there are newer ones
which will operate with only half a volt difference.)
Note that adding a resistor in series with the input of the regulator
will shift which element dissipates the heat, but doesn't change the
total amount generated.
For over-voltage protection, the standard approach is to use an
SCR "crowbar" to blow a fuse when the output voltage exceeds
a certain level. (When the voltage on the SCR gate get high
enough, the SCR conducts and shorts the output side of the fuse
to ground.) However, there is a finite time required for the SCR to
trigger, and it might be a good idea to add a large 16 volt zener
across the output to keep the voltage from damaging the equipment
before the SCR triggers.
Good luck! - Dale WB6BYU