Need help, What is the biggest size of cam I can run?

Talk to a comp cams rep or something like that. If you are really looking for the biggest cam your 350 can use, you need to blueprint and flow your heads so you don't run into coil-bind and other problems. You should go with a ready made package (edlebrock has a few) for a stock motor if you don't want to do any thinking. if you want a generic, loping, performance cam try something with a duration of 230-240 at .050, an intake centerline of 108, and stick with a stock-ish lift # to avoid problems with coil-bind, valve to piston clearance ect. Just remember, if you have an automatic trans you will most likely have to upgrade to a higher stall converter.

If you replace a stock camshaft with one that gives the engine a "lope" you will have to replace the valve springs even if the engine was new.

There is a lot more to it than lift and duration of a camshaft.