Can anyone here tell me how much labor it will cost to put a new 12 bolt rear end in my 68 chevelle? It has a factory 10 bolt and I wan to swap it out.

I'd say a good shop should have it completely changed out within 2 to 3 hours (one man). That's if there are no problems with rusted brake line fittings, worn brake hardware etc. As far a price, it depends on your local shop's hourly labor rate.

you might want to consider doing it yourself. It is actually a relatively simple task...with the exception of the "rusted brake line fittings, worn brake hardware etc." that was previously mentioned. You'll need some simple tools like floor jack, jack stands, wrenches etc. but it is a good excercise in swapping stuff. Save the labor cost and spend it on tools and a shop manual.