Using 8 3/4 with /6

welding them how long would a 7 1/4 last ?was thinking of doing that myself would this effect over all mileage? i would think it wouldnt just maybe it would'nt handle corners that well?
If you live in an area with a lot of curvy roads, you will definitely see a hit in gas mileage.. And you want to go real easy on the rain.
How long will it last? How well can you weld?