Line boring has fallen out of favor since most blocks don't require it anymore. You can buy new blocks from a number of vendors that are ready to go with just a line hone. Putting new caps on a block and boring the main line is rarely done anymore since it is expensive and time consuming. So I'm not surprised that you're finding it hard to find someone to do it. The sad thing is that your customer's block probably didn't even require those aftermarket caps. Stock caps should work okay on a 383, even if it is a stroker engine. But that is most likely water under the bridge by now.