I totally agree with Sweet, drive it a while before digging into it. Once you replace the engine, brakes are next on the list. The rears will come with the new axle, but the fronts will need changed out to match the bolt pattern of the rears, unless you want to carry two spare tires. Most replacement axles will need the spring perches changed out to the narrow 1 3/4" ones to match the springs. If your truck has 2" wide springs, then most you will have to do is relocate them. You will also have to mock up the axle to set the pinion angle once you cut off the spring perches.
My '37 truck has a '79 Firebird rear axle. It fit the width just fine and gear ratio options wide open. I chose a 3.08. In my case, the truck will never haul a heavy load so the axle weight will never be tested. If you plane to work your truck, check axles weight capacity before installing. The Blazer 4X4 should be about the same as the 3100, so it makes an excellent chose.
The power glide in my opinion is a very poor choice for a transmission. A turbo 350 is a direct replacement for the power glide. You gain a third gear and a lot more modern transmission. The power glide will need 3.23 to 3.55 gear ratio to get the truck moving at a decent rate, but then you are stuck with high highway rpm. The power glide will act like a T350 starting in second gear. Go T350 with a 3.08 or 3.23 gear and you will be happy. Tire size matters, so figure that in as well. 235/75R15 radial is 28" tall, thats about the average tire size. If you want overdrive, a 200R4 is made from a T350 and will also fit.
Joe
