Vacuum leaks can be checked by first visually inspecting vacuum lines, then spray them (while running) with carb cleaner, brake clean, water, etc and listen for any changes in the idle. If so you've found a leak.
Why would you think the clutches are worn out in the trans? 4t65e's have gone much further.
To fix the gas gauge you will likely need to replace the sending unit inside the gas tank as the teeth on them wear out. I was having the same issue and that fixed it. (Part#: 911-008) $20 http://www.ebay.com/itm/Chevy-Buick-...f19369&vxp=mtr
