Hi, I built my 3.42 posi on a budget for my 79 malibu wagon.
First, I got a posi unit. You can get ANY unit. It doesn't have to be 3.23 and higher carrier; but if you can find one for 3.23 and higher, get it. It's better suited, and the lower numerical carriers require a spacer.
I got a 2.73 posi unit out of a 1988 IROC Z camaro for free. I only took the carrier, not the gears. Then, I scored a 3.42 ring and pinion out of a late model camaro on ebay for like $40 shipped. I bought a spacer to adapt the high numberical gears to the low numerical carrier (not necessary if you find the 3.23 and higher posi carrier). All bolted up and set up with an installation kit and seals I probaly only have $165 or $200 in this entire setup.
It works great. There's no way I can break this thing unless i use slicks AND hook it up. That's an entirely different post; and the main reason G body owners go for the 8.5 GN rear, ford 9", or Strange 60.
For budget fun, this is the best bang for the buck to improve your street driven malibu. Going from the lousy 2.41 gears up to a 3.73 or more will really make the car a lot more responsive when you're driving around town.