The 6308 are stock Z/28 springs and are about as soft as you would want to use. Since you will use stock type wheels and full tread tires, not special autocross tires or extremely wide tires, this spring would be a drop in and work pretty well. It would probably allow a little excessive travel when you get into hard cornering and would allow a little more front end lift at very high speeds.
#5536 will hold up very well on the street and still give a good ride, will limit bottoming out if your car is lowered, will not allow as much front end lift at high speeds.
The 5536 are pretty close in height but are a hundred pounds stiffer than #6308 and one end is different than what is needed for a Camaro.
You will have to cut the different end to match a Camaro spring, the other end is OK as is.
I'm guessing it might need to cut to be an inch shorter in standing free height than it comes, to equal a #6308 in height, but that is just a guess. It is listed very close to the #6308 in free length but is stiffer, so would sit higher for a given free length.
pdq67 found this one and I don't know if he has installed them yet. I have no experience with them.
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