the blazer should fit.
i have two friends that fitted standard blazer rear axles on their trucks.
I have a full size suburban rear end under my 3104 and it
fits just fine .
There are lots of rear ends available! Some fit better than others. For 6 lug rear ends, the 64-69 1/2 ton 2wd Chevy (and some GMC) trucks have a 12 bolt rear, about 60-61" wide (I measure them where the wheels bolt on). The 70 2wd and 70-81 4wd have a 6 lug 12 bolt that is a couple inches wider, but will fit under an Advanced Design or Task Force truck as long as you don't get too wild with wheel/tire width. The heavy duty models have a 2.75’’ brake shoe, that’s stopping power. The 12 bolt truck rear ends from the 60s usually had 3.70 gears, which is a bit much for extended highway driving at 75 mph. (unless you hgave a 5 speed), You can swap the differential carrier and ring/pinion gears from a later TRUCK or VAN 12 bolt, the 70s trucks usually had 3.08 or 3.40 gears. Of course, you'll need to know how to set up ring/pinion gears, bearing preload, and all that other neat stuff to do this. These 60s truck 12 bolt rear ends are from coil spring trucks, so the perches are in the wrong place, and they have some extra stuff on the housing for a panhard bar, which you won't need with leaf springs.
BUT
don't take my word on it
see: chevy trucks. org
http://www.chevytrucks.org/tech/rear_ends.htm
