The two hole hanger is throwing me off a bit. That sounds like it could be an SS hanger, but an SS spring will lift without traction bars or a snubber. If the snubber was slamming the floor and the slappers are lifting the car, that tends to point to standard or XHD springs. At this point, the only sure way to know is to break out the tape measure and check from the axle locating bolt to the center of the spring eye. This measurement will confirm what the springs are, or if you can post up the summit part numbers of the leafs since you have the reciept.
KYBs are an okay shock, but their high pressure gas construction may not lend them to be a good strip launch shock. Sounds like they have adequate travel through and the system should work with them although it may not be optimal.
Invaders are a 380 UTQC rated tire. That means it is a fairly hard compound. Great for mileage, bad for traction. A good performance tire will be around 200 or under in this catagory. Race tires actually carry a rating of 0. Also, radials do not respond to air pressure changes the same as a bias ply tire does. As you noticed, once a radial looses traction, there is no regaining it until it stops spinning. They also break traction suddenly without the broadcasted warning of a bias ply. This is why your not seeing the improvement you were once familiar with when airing down. If your serious about hooking all the time, you may need to put the ET streets on. Otherwise accept that the street performance will suffer because of the current tire compound.
Also, you can try removing the furthest most leaf spring clamp of the rear segment of the springs. This should soften the hit some and allow the rear of the pack to flex a bit more which may give the tire time to hook. You can mix this up by alternatly adding clamps to the front segment to stiffen it up, like CP suggested. Mix and match here until things start to do what you want.