Potential swap from 13 to 18?

Greetings! I'm currently restoring a 98 Classic and am considering changing the transmission. Why? Well because it's out right now and it's pretty much the only thing on the whole truck that isn't already being addressed. Half the labor of a swap has already been billed.

The transmission is an RTLO16713 with about 300k miles on it. I'm sure I could sell it no problem.

Power plant is a 12.7 Detroit flashed to 500hp, currently undergoing full rebuild, and the rears are 3.58 with LP22.5. This truck will be running 105,500# in W. Washington, not going over the big hills.

The question is, would there be any benefit of swapping to an 18, and if so, which one should I consider (I split the bottom end in traffic to stay at a good rpm). What about moving to 11R22.5? How would that impact things?