i have been reading a lot about moog, and yes the 5658/ 5413 has been thrown out a lot. and i have also been interested in the umi rear 1" let me know what the out come is. i'm not afraid of stiffening up a bit, but i do like a comfortable sunday drive. i am going to keep searching.It seems most list the 5658 fronts, 1" drop 530 lbs/ in. spring rate and the 5413 rear, 1" drop 167 lbs/ in.
These may be a bit stiff for you, however I like more of a Z28 or GTA ride and have had some ATR performance handling springs in the past. Although not listed, I think by the wire diameter the springs were in the 530 lbs/ in. front and 125 lbs/ in. range and even with bilstiens I thought the front was a little soft.
The 167 lbs/ in. rear springs may help plant the tires at the strip, but I feel they would throw off the balance of the car handling wise.
I would not be afraid of the 530 lbs/ in. front springs especially with softer shocks if you could find some rear springs in the 120 lbs/ in. range.
I am trying the UMI Performance 1" lowering springs that are 600 lbs/ in. front and 130 lbs/ in. rear.
I'm making several other changes that seem to be taking forever so I don't have any drive info yet.
Another thing to consider is old stock springs have sagged an inch or better over time and new lowering springs may leave the car the same height or taller than the springs you have now.
Here's a link to the Moog spring chart. https://www.moog-suspension-parts.com/universal_coil_springs.asp
You can scroll down by part number and look up the specs.