My thought processes are going down similar paths as Brent and Gary. If, as I suspect, the other bearings are not spun then you don't have an
oil delivery problem. The bearings farthest from the
oil supply will always fail first, not last if you have an oiling problem.
Something awry has happened to that rod bearing and possibly others. When you use a full radius crank you need to use narrowed bearings. Many times the bearing manufacturers will offer such a bearing and other times they do not. As Brent suggested the side clearance also looks very tight. When you assembled the engine where did you set the side clearance?
My suspicion is that you used a standard rod bearing and it was too wide to clear the radius the crank grinder cut into the crank. FWIW full radius cranks are stronger than the same crank with an undercut radius but, you need to use narrowed bearings to clear the radius or the bearing will spin in the rod.
Bearing manufacturers make narrowed bearings for common race applications usually SBC, BBC and Hemi engines and an occasional other. If you use a crank that has full radius journals and your bearing manufacturer of choice does not offer a narrowed bearing the job falls to you to narrow the bearing.
Lets say your crank uses a 0.100" radius on the journal, that means you should narrow your bearing width 0.100" on the side of the bearing closest to the radius to clear the radius. Rod bearings are usually set back slightly from the radius facing side of the rod. The minimum amount you will have to trim a bearing to clear a radius is the crank journal radius less the bearing offset in the connecting rod.
Crank manufacturers vary the radii they use usually between 0.100" and 0.125" depending on the crank's intended usage. The 0.125" radii are usually reserved for supercharged Chrysler and BBC cranks and the 0.100 radii is used for most everything else.
When I did a quick search of my Mahle (Clevite) and King Bearings catalogs I did not find any narrowed bearing offerings for 427 engines. That doesn't mean they don't exist but it does mean you would have to make a special effort to find them or make them yourself the way we did decades ago.
My bet is you used regular of the shelf bearings for the assembly and one was just too close to its crank journal radius and made contact. At that point the rest is history.
Your rod journal might be salvageable without cutting the crank. Your crank manufacturer could better advise you on that. If the failure was because of OEM bearing widths being used with a radiused crank journal then, I suspect there may be other wounded journals as well. If there are, you will find them as your disassembly continues. In the FWIW bucket, it would be interesting to see what the sister bearing on that journal looks like as Gary has already asked.
There are other plausible explanations also but standard width bearings on radiused journals almost always have an unhappy ending.
Ed
p.s. the big end on that rod will most likely require resizing to get its proper diameter and bearing crush restored.