The full difference need not be shimmed. The correction, depending upon the source of the leg length difference, is to shim 1/3 to 1/2 of the distance. 1/4" to 3/8" is still pretty significant, but could be made to work with an insert under the foot in addition to cleat shims, or perhaps some other solution. I'm not a fitter, but my understanding is that different length cranks aren't generally considered a good solution. Maybe with such a big difference that changes things, I don't know. I will say that I also have a difference in leg length, though it's more like 6 or 7 mm in my case, which is fairly mild. But that difference has caused me injury problems in the past. The OP's leg length difference is about 3 times more severe than mine.