Hi Ceefna, looking at it again the LM317 or LM338 might not be the ideal controller as normally you would use PNP pass transistors with one, and you already have 2N3055 NPN pass transistors. Instead I would use a single NPN near ground as the current regulator, turned on when it gets >0.6v on the current sense resistor.
That should be extremely rugged and stable enough, but it has the drawback that current is set by the choice of a power resistor. Or if you want to use a pot the current adjustment is limited to about a 3 to 1 range via pot adjust. Of course you can have "ranges" selected by switching in different power resistors, so with say a single switch and two power resistors you could have two ranges;
low range = 0.5 to 1.5A
high range = 1.5 to 4.5A
If you are happy with that type of limitation then I'll rough up a circuit. Basically the main improvement you get with this is removing anything sensitive like an op amp, no need for regulated supplies to drive chips, and the entire electronics is in series with the load and on the low side, so only the power transistor collectors are exposed to the anodising tank and any shorts/spikes etc.