I'm trying to sell this rifle BUT if I can't I want to build a 257 Weatherby Mag. Would I have any feeding problems or any problems? I was thinking on putting a Lilja #2.5 barrel with a 1-10 twist.

If it feeds fine as it sits, there shouldn't be any reason that a feeding issue should crop up on you with the 257 build, providing the mag box is long enough to seat bullets where you'd like them to be......and that's an unlikely problem.