Advice on camshaft for a 318, please.

There pinion size's are different. They will all handle a preety high horse power load no maatter the size.
For what you need it for and at the power level your at, there is no worry about what pinon size to use. Just look for the correct U joint size to match what you have now or you'll end up with a cross over joint which is a joint of 2 sizes in one.
Also count splines to match it with your axles. Just to be sure.