Turbo 383 in a A-body Barracuda

been al over the internet looking at rear sway bars and all of the ones for these cars will end up hitting my coil over shocks but got to looking at the nascar style ones they seem like they could work even found a welder series for cheap but have reservations on them because i dont know how to figure the spring rates on the torsion bars does anyone here understand how to figure the spring rate a person would need