Can you put a LLY in a 2001 truck? If the LB7 blows up and you need a new one can you just put a LLY into the truck?

The short answer is no. It would require a PCM and wire harness change...and oh yeah, a new Allison to match that new harness.

The short answer would by yes. Fit is not an issue, that part would be easy. However several other things would need to be changed. Wiring harnesses, computer modules, I doubt the Allison would need to be changed out. Most likely just updated. With it being an 01, I'd guess that you'd have to change quite a bit more than you would an 03 or 04. You'd have to update the truck first. I don't think this would be very hard, just expensive and time consuming. It's the equivalent of upgrading the LT1 to an LS1 in a camaro. Again, very possible, but for how much...
I'd just go for a truck with one already in it. When your truck has seen enough miles to kill the LB7, there will hopefully be used LLYs out there. Then buying a used one with an LLY in it would be much simpler and less expensive.

Ox