You had run this crank, this block, these rods, at these clearances, with thicker oil and likely same type filter in the past with no issues. So WHAT changed. You heavily modded the oil passages, but you did a lot of what is standard procedure on many small block builds, so that shouldn't be an issue, unless it is broken through somewhere you aren't seeing. Now you say the rear cap is broken. That could have happened first, or as a result. If it broke first, would be a problem and usually a broken crank does that. Crank MAY be broke, will have to Mag it to see, IF you even care. If crank is broken, my GUESS would be the root of the problems...........but sometimes shizzit just happens and you never really know what happened first. If you are confident the machine work was right, your mods were sound, the clearances were right, it was assembled correctly and it just broke. Accept it for what it is, a bad break and move on. Don't let it make you question what you did or how you did it. We all want to say "this did it".......well sometimes you just don't know. So don't pin it on oil, filter, or anything else, unless you KNOW. If you have built several and then you break one, doesn't mean you don't know what you are doing or even did something wrong. Just keep on keeping on and file it as "bad break". It's happened to about everybody