The discoloration above the oil ring on the piston is extreme. Its getting combustion leakage past both compression rings and oil from the other side. Its not sealed and I don’t believe it ever was. The cylinder finish was incorrect for these rings, plain old cast rings would have worked better. Do you know the cylinder/piston sizes yet?

I get opinions will vary and every body has one Monte….but a few key points here based on the photos and circumstances of what the OP presented.

1. It turns out to be only a 446….in no way is power killing that block with those heads and that cam.
2. The photos do not support enough evidence of detonation damage to crack this block. Detonation will leave some physical damage to other key parts if it has the energy to crack a block.
3. Filling a gray iron block and filling a nickel, ductile fortified cast block yields different results, gray iron is about the worst casting material out there for high stress.
4. Filling to the bottom of the freeze plug does squat for strengthening the cylinder and helping ring seal….the top ~2” is what needs the help. That low fill isn’t helping with ring seal or making power in any way……but what it is doing is reducing the ability for this thing to flex/move…..inducing stress failure.
5. Hard block is a good product…but read line 4 again……
6. This thing evidently cracked when fired up from winter storage….pretty sure the power/load wouldn’t make the grade for any type of detonation to occur – lol. Thermal change when fired up from winter storage, possibly at a quick rate is what I believe cracked this block.
7. Aluminum caps ain’t where it’s at……but we can leave that as my opinion only……

6bblFLASH; You have several issues here to work with and make significant improvements if you so choose, but honestly – I would simply start fresh with a good block and go to a reputable shop specializing in PERFORMANCE machine work. The precision is worth the cost. I agree, It DID perform well for the components involved and even considering the other issues, I think it will probably surprise you when its all fixed. This probably doesn’t need mentioned….but make sure those pistons are not dimensionally damaged. You may find some rolled skirts, if you do, don’t match bore the new block to those pistons, start with all new stuff, cheaper in the long run.
Good luck, maybe see you around the Ohio area.