There ARE disc brake spindles available for the T-bar front suspensions.
As far as modifications...are you looking to use new ball joints with the stock control arms? Won't work.
I've got a '62, and am in the same situation as you: this is my thinking on it...
The torsion bar front suspension is about the most comfortable truck I've driven in. Ball joints are horribly expensive, but then again, so are the ones on my new Dodge Dakota.
If I spend a truck load of money to rebuild the front suspension, how long will it last? Probably about 100,000 miles...which in my case is 5-7 years. Then it's time to rebuild again.
During that 100,000 miles, I'll have a comfortable ride.
My one concern is replacement torsion bars, because I haven't found anyone that carries them. But I'm working on that.
Now, to swap in a later cross member, I've heard it's very easy. Initial cost of the new cross member, steering box, total rebuild and rebuilding the disc brakes (another reason the swap is so popular--factory discs) is probably going to cost you what rebuilding the stock set-up is, with the benefit being cheaper ball joints during future rebuilds, cheaper coil springs, more commonly available upgrades such as disc brakes and sway bars (I'm working on sway bars, too).
The short money is going to be pretty close. The long term money is going to be a bigger savings, just in the cost of the ball joints.
I have to buy expensive ball joints for my new truck, so I got over the dread of buying expensive ball joints for my vintage truck.
-Brad