Outer wheel bearings in all Dana-Spicer rear ends are fed from the round hole located high on the drivers side. That Inverted V stamped into the rear cover isn't to provide structural strength, but capture gear lube slung off the ring gear. The housing is machined to feed both sides and the front (driveshaft side) pinion bearing from internal oil passages.
That said the SAE 90 weight gear lube (or 85-140 multi grade) oil isn't going any where until it heats up. As you can see with the plastic rear cover to watch the inside once it reaches operating temp it flows like water.
NASCAR uses a positive displacement oil pump, driven off of a pulley bolted to the rear yoke, to force oil to the bearings after it goes through a cooler because at 140 to 200 miles an hour it thins the grease out to the consistency of a light lubricant similar to WD-40 after hours of racing. A light oil isn't a way to lubricate roller bearings.
Further I never was on the clock as an employee at my local Chevrolet dealership from the mid sixties to the mid eighties I practically lived there. If you drug a car in for a warranty claim on a broken rear end and they saw an aftermarket cover on the differential the warranty writer would automatically void the warranty due to lack of oil. Even if you broke the ring and pinion drag racing, just the cover meant you bought a new rear end.
Finally in 1972 GM switched from Spicer as the vendor for rear ends to American Axle Works (the folks that made the 14 bolt rear to replace the Dana 60 under light trucks, and the 7.5 inch ring gear found under the Vega). Their "Corporate 10 bolt" better known as the 8.5 rear has no stamped V to divert gear lube to the bearings. I actually have no idea how it lubricates parts so I can not comment. But since it is flat rear cover might be fine with this rear end.
I never used a cast rear cover with the preload bolt (from an engineering point of view it is a good idea). Instead to reinforce my 12 bolts (back when I still ran a 12 bolt was to replace the cast iron saddles with steel caps and then remachine the rear end after welding the tubes solid. The larger thicker bearing saddles where stronger than a stock cap and a cast cover. After my third modified 12 bolt failure I switched over to a Dana 60 and never had a broken rear end again.