Tips for HP gain?

I would not relocate the springs. I would spend that money on something else, maybe a converter. Not an off the shelf one. You can fit a 8-9” tire on these cars without changing anything. A 8” M&H muscle car DOT race tire is enough tire for what you are doing. I run 7.50 in the 1/8 at 91-92 mph on a 6” repo redline tire with NOTHING special in the rear suspension.
You need to stop the spinning tire first. Which you can probably do for free.
Technique, no long burnouts. Just clean the tire and you are done! Big burnouts put more rubber on the track, but do you no good. Tire pressure, go some place safe and spin the tire, look at the black marks. You want a good ever colored mark across the entire width. If it’s light in the middle go up 2 lbs or air pressure. Light on the edges, go down 2 lbs. repeat until you get ever color. The pressure will get higher as the tire heats up. So let it cool between tests. Launch rpm, are you launching at idle? If so the converter might be flashing too high. Trying loading the convertor so the engine doesn’t have a “running start” at the converter. I launch at about 3000 rpm If I go lower it knocks the tire off. My converter stalls at 3400-3500 rpm. Don’t remember what it flashes to. Also pay attention to the “minefield” of water around the water box. Look to see if there is standing water in front of the water box. There usually is because all the burnouts create grooves in the surface where water collects. Line up offset from the grooves. If you can, drive around the box and back in to just wet the tire. Roll all the way out of the water box. Quick spin first hint of tire smoke and you’re done. One of the most useful things to do is have a friend use a smart phone to slow-mo video the cars launch. You will see things you would never see with the naked eye. If I can figure out how to post a video, i have a slow-mo of a 1.73 60’ from last weekend.