To give you a good idea of what delineates a slant car from it's hemi and six pack sisters, here is a photo of the underside of "El Hemi" owned by the Escalante family. El Hemi became legendary on Woodward, then made it's way to Argentina, and then back to the states...
Those are your torsion boxes to the extreme right of the photo. Also, take note of the extra metal above the pinion snubber area. Hemi and Six Pack cars got those.
Other things they got:
*K-frame with skid plate
*3/8" Fuel line
*Bigger Torsion bars (7/8 versus 13/16, I think, but that's already been covered).
*No A/C.
*Torque boxes
*Pan over pinion snubber
*Extra half leaf on passenger's side spring pack.
That's all I can remember off the top of my head, I know I am missing a few things (big radiator, I think there is structural reinforcements elsewhere) but a subtle thing that will carry you a long with these cars is extra welding. Remember that some stiff with an a-hole of a boss was slapping these things together! I remember seeing a thread on here of a '71 Barracuda (I think) that showcased some beautiful welding in mostly factory locations. That will stiffen that unibody pretty good!
Later
KFD