How hard/expensive is it to switch out gearing on an A-body?

Pulling yours and have it rebuilt with the right gears probably the best way to go.

I want the opposite mine came with 2.96 gears and I want 3.91 but want to keep the 2.96 intact for when I do long trips. So I'm looking for a complete 3rd member.
Up here a fully rebuilt 3rd member to be purchased out right is about $1600 :(