550-600HP 4spd suspension help

#5 What would be a good starting point for rear suspension for a full interior 69 swinger.

#5 ladder bars with spring sliders on your stock springs and good adjustable
shocks. cost about $500+ or -

I like Bob's idea here, especially on a high powered stick car. If you use floating spring mounts, like these,
View attachment 11119.jpg

you isolate the spring from the housing (basically they become car support mainly) and you use the ladder bars, or a four link to adjust the instant center, further forward (down) most likely here, and that will allow you to not hit the tires any harder then is necessary to get a clean launch.