Sounds like you're fighting more than worn bearings. I'm thinking you don't have enough radiator. Don't know if it's due to an old and dirty radiator, or that these trucks were marginal on capacity to start with.
On your oil, don't be afraid to try something heavier. Around here I can buy SAE 40, sometimes SAE 50 if I want to look hard. If your air temp only drops to freezing, you shouldn't have any trouble starting with SAE 40. It's worth a shot at the next oil change.
I was looking at some of your other posts seeing you want to build a 261. I would speed that up if I could, just in case this engine doesn't make it. But you will need more radiator for that, more power needs more cooling.
You never know how long an engine will last under these conditions. I had one six in a 5 ton that had good oil flow, but little pressure. We put a few thousand miles on that engine, running wide open almost all the time in a fire truck. It could have gone at any time, but it was still running strong when we sold it.
