I called Chevy Duty this morning. Their 3.55 gear set is described as "bolt-in". I spoke with the tech and he said it was an easy job, it just took a long time. Well at $75 an hour, I don't want my mechanic doing "easy but long" work.
We found a used diff in North Texas for about the same cost as the new gears ($400 plus shipping). Thanks all for the advice. Now I've got to get the speedometer working again.
Bill