Hello jiffy,aha - ok, my take then (based on hope, intuition, ignorance and spending avoidance) leads me to think that if I leave the 28-spline axles in, then the bearings will not be particularly weaker like they would be if I had the 31-spline axles in this housing. I could upgrade to the 31-spline axles so that the axles would be stronger but then I'm transferring the strain to the (even) smaller bearings.
I'll not be racing this thing, and it's always likely to be traction limited, so the amount of stress I put on the 28-spline rear shouldn't worry it too much, and as the thing has literally JUST (like 50 miles ago) been rebuilt, it should be fine for street driving and if it breaks, it breaks.
At the end of the day, it all comes down to maximum torque differential on components. You can put a high horsepower 427 with an automatic and regular width street tyres in your car with a light duty axle and it will probably live. The narrow street tyres will spin lowering the torque differential and parts live longer. Now if you put wide slicks on it and have a manual transmission, rev up the engine and dump the clutch things are going to go flying and it won't be the car. Under those circumstances the instantaneous torque differential on all the components is astronomical in that situation. The same is true with automatic transmission shift kits, put hard hitting shift kits in an automatic transmission and drive line life is greatly reduced.
It really helps to look at the big picture of the car and intended use on judging what should be upgraded.