Anyhow, I too realise that the TO-3 package is vastly different to the TO-220 package, but apart from the obvious issues mounting a TO-3 with heatsink to the PCB, it is still an easy task to substitute the LM338K if it will work in the circuit.
I'll try and find the relevant PDF when i'm at work again in the morning.
EDIT: Oh bugger! I remembered where I read it, and it wasn't even a relevant PDF. It was in the datasheet from National Semiconductor! Clearly i'm getting old! Quoted below.
For applications requiring greater output current, see LM150
series (3A) and LM138 series (5A) data sheets. For the negative
complement, see LM137 series data sheet.
FURTHER EDIT: In support of my other question, here is another quote regarding the fact that the max voltage handling capabilities of the LM317 refer to the differential between the input and output voltages, and not the maximum voltage above ground that the regulator can handle -
Besides replacing fixed regulators, the LM117 is useful in a
wide variety of other applications. Since the regulator is “floating”
and sees only the input-to-output differential voltage,
supplies of several hundred volts can be regulated as long as
the maximum input to output differential is not exceeded, i.e.,
avoid short-circuiting the output.