upsrodder,
I'm not expert, and I'm sure if I'm wrong, someone will be along real quick to correct me.
I think the mono's will lower you about 3 inches. Lower, will likely require a dropped axle.
I'd probably go with the Mustang II. If it isn't low enough, you can always buy dropped spindles for it and really drop it down.
You'd probably have a nicer ride with it than the mono-leafs.
I can't think of any reason why you couldn't keep your 235 with a mustangII front suspension. Your 56 block should have side motor mounts.
Since you don't have front motor mounts, you don't have to worry about anchoring the block to the cross-member, like the earlier blocks did.
Only thing I could think of, is that you'd have to fabricate a mount for the radiator support.
Again, there might be something I'm missing, but it seems easy enough.
