If this is a Shadow II (which I think it is) or a late Shadow "One" you can fit Kumho Solus KR21s in P235/75R15 with absolutely no problem at all.
As of about two or so weeks ago, there is absolutely no one, including Avon, currently producing whitewalls in the originally specified size. (Unless you can find existing stock in some warehouse, then you have to be careful about the age of those tires).
One nice thing about the Kumhos is that there are two load ratings available in that size. I went with the lower of the two, 105, since that's more than sufficient, but they also come in an even "heavier duty" 108.
No matter what tire you choose, do not use the originally specified inflations, which are woefully inadequate and so low as to be unsafe. I keep mine inflated to at least 30 PSI in front and two to 4 PSI more in the rear.