I would use either the Hotchkis or GW rear springs whichever is stiffer.
The Hotchkis are around 175 lbs/in according to my testing of them which is difficult to do reliably.
I feel the Hotchkis may be too soft for the power you have, and the power I will be having in my 67 too. I needed something to start out with and chose the Hotchkis as a good starting point.
I autocrossed with Guldstrand rear springs which worked GREAT!
The only problem is they have bronze bushings in them that rattled my teeth.
I don't have a rate on them to compare but will when I get them off the car.
Front to rear springing is a very important balance.
I had no wheel hop on my 67 at all and it was great after having mono leaf spirngs on there.
If you use the Landrum adjusters, use the springs that go with them that arre 9.5" tall, 5" dia. they are flat on one end to fit the adjuster.
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327 Origonal owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer