Gr8lite
The Posie's springs are good, but they're expensive IMO. With a little fab work, you can build something just as good and use that money somewhere else.
I used common trailer springs on the front of my car. They were cheap, but needed some work to make them look good. I rounded the ends of the leaves just for looks. The spring eyes were also a little smaller in the ID than the OD of common shackle bushings. I had to turn the bushings down a little to get them to fit. The springs are working well, even though they have only 3 leaves each. My car carrries about 960 lbs on the frontend.
If I was doing it again, I would probably use something like a 32 Ford spring. It's long enough to make 2 springs, has thinner leaves (but more of them) and already has rounded ends on the leaves. Just more expensive than the trailer springs.
I mounted my springs under the frame rails. This just seemed a little stronger than hanging them off the side of the rails. If you do mount them on the side, make sure you build the brackets heavy enough to resist the twisting forces on the spring. Not much room for gussets or other bracing on the side of a typical 3" or 4" frame rail, but not impossible either.
Mike