You're correct in thinking that at a quarter million miles the trans could be on borrowed time however the engine, t case, axles could be as well. Although, interesting fact that the 4.8 and 5.3s are some of the most bulletproof engines out there. They actually hold up to serious hp with stock internals.
If you don't have any issues currently personally I'd save the $ for now and run it and maintain it as you have been. Couple thousand would go a long way towards a newer truck and at the miles you're at, not worth putting thousands into it IMO unless absolutely necessary.
Ditch the 6 speed idea. Not feasible.