Fixing very old, very undersized ceiling joists
You have several challenges when adding many thousands of lbs. of weight to a roof system like that (which almost certainly wasn't designed with that use in mind):

The ceiling joists aren't up to the task, as you know. The only real fix for this over a 24' span is to tie them to the rafters, which leads us to the next problem...
The rafters are barely adequate as they are. If you now hang the weight of a gypsum ceiling on them they'll soon be looking like your ceiling joists do, or they'll just give up the ghost. Unless...
You replicate a modern engineered truss with V-webbing and adequate gussets at all connections. They'd look about like this when done:


source
Note that gussets must be fastened properly--those spiky steel plates cannot be simply pounded into place. They're not designed for that. If you were to screw or nail plywood gussets on, they'd need to be robust enough. Wood isn't as strong as steel, so you need a larger panel of suitable thickness.
As to your strongback proposal... a strongback stiffens things by sharing load across several attached members and keeps them on plane. It does not hold them up. You'd have to install a beam that spans the 60' down the center of your building while carrying nearly all the weight of your insulation and drywall.
I strongly urge you to get a local expert on scene. This is a project that could get dangerous in a hurry if not done well. Good luck.