To Many unknowns here. 30° initial is to much. I know this is guess, but if everything is matching and balancer ring is true, you should still be on the timing tab. 16° btc would be max initial base timing for a normally aspirated engine, this is with a properly tune and jetted carb. 10-14° is more realistic, still with a properly tuned and jetted carb.
Something is not right here. I believe you when you say the engine has more power at WOT, with set at your estimated 30° initial timing, thing is it shouldn't.
It should ping and rattle get hot, and have slow crank cold or hot. So we need to find out what is going on here.
1ST thing we need to be sure of is the timing mark and tab are accurate. With out doing this then you might as well guess and time it by ear, cause using a light on unproven marks is no different.
Step one get a piston stop tool.
Or make your own from an old spark plug. Knock the porcelain out the plug nut, tap the nut 3/8 16 tpi 2" 3/8 stove bolt.
Step two remove #1 spark plug, front cylinder driver's side, install piston stop. Turn crank slowly by hand until you hit the piston stop. Mark Balancer at 0 notch on timing tab. rotate the crank opposite slowly by hand until you hit the stop again ( do not allow the stove bolt to move between directions) mark the balancer at 0 on the notch timing tab.
Step 3 the exact center of the two mark on the balancer is true Top Dead Center #1. Mark this with something that will show under the timing light. If the center is the timing grove on the balancer ring so much the better. If it is not then your balancer ring has spun and is no longer a dependable timing mark. Nor is it balancing the crank shaft properly and should be replaced.
Once we have a proven balancer mark we need to match the balancer with timing tab. 2 style of tabs where used. 1st and most common is with the tab at @ 2 o'clock. 2nd the tab is at 12 o'clock.
Step one remove piston stop, either use a remote start button tool or have a helper. Stick your thumb over the spark plug hole, bump engine over until the pressure pushes your thumb off. This will get you close to #1 TDC Compression stroke.
Step 2 reinstall the piston stop, screw in the center bolt, rotate the crank clockwise, looking at it from the front, until it hits the stop. Back stop center bolt off a turn and rotate crank until it hits the stop. Do this until you are close to lining up the timing marks. Once the marks are close, say 2° the piston stop may not hit. If the Proven mark on the balancer lines up with the timing tab 0 notch your good to go. If it does not then you need to mark a second TDC, to match the timing tab 2.25" from proven mark we established earlier. Or get matching parts. Since your engine is SBC 400 I am going to go out on limb here and say there is only 1 choice, and that is 2 o'clock timing tab position.
Once all of the above is done and proven, we can use the timing light to set your initial timing and check total timing, both mechanical which should add @ 20° to your initial timing setting, and with vacuum.
