If your shooting for 350rwhp it will be tough to make with the stock 5.0 shortblock. It's possible but you will have to have a very well put together combo. The trickflow heads are pretty good but I think there are better cams out there than the X303.
A well known combo for a 300-330rwhp is AFR 165 heads, Edelbrock performer intake, FTI custom cam, and all the supporting mods.