[Mike C]

Originally Posted by Ironangel

Your existing carrier needs to be 3:73 or up. Otherwise, the 4:10 wont fit unless you install a "thick" ring and pinion set on 4:10 posi carrier. Scroll down to 12-bolt truck... http://www.differentials.com/technic...arrier-breaks/

I guess he didn't say if he was changing gears as well so I will assume he is going with the 4.10's and the posi from the Blazer. I would also hope that this is an aftermarket posi and not a 12 bolt "Gov-Bomb" that would have been factory in a '78. If it is the Gov-Lock it is most likely a 3 series with a thick gear.

If the Blazer is a 12 bolt you can swap the parts out. Buy the book on rebuilding 10 and 12 bolt Chevrolets. Check the pinion depth in Blazer 4.10 unit. Check the backlash in Blazer setup. Remove c-clips and pull axles. Remove carrier and ring gear. Remove pinion gear. Put pinion gear in '70 housing. Check pinion depth. Adjust to match what it was in Blazer housing. Put carrier and side shims in same as they were in Blazer housing. Adjust backlash.

BUY THE BOOK THAT TELLS HOW TO DO THIS AND SHOWS THE TOOLS YOU NEED!

I encourage everyone to do as much work as they can for themselves. But this job needs a little more info than you will get from asking on a message board post IMO.