Bill...I'm pretty sure this is how it works;
(from Greg Roberts)
69 L78 HD F40 had BK coded 5 leaf springs 100 pound rate(Listed as RPO on the spring calculators)
69 L78 Special Purpose F41 (springs and shocks only) had BG coded 4 leaf springs 125 pound rating (listed as HD on my spring calculators on my site)
more info here:
http://chevynova.org/rearsprings.html
There was never a sway bar available in 1969..this started mid-year 1970 and was part of the F41 suspension package.
***there are some really good aftermarket sway bars that look "stock" when installed...we have one on our 68 BB Nova...along with gas shocks and radial tires. The sway bar made a huge improvement in handling and eliminated the body roll from the old setup...I think it would work well on your project...
tom