there are a number of things you need to do to the 351c heads to get them to work on a windsor block. as stated you need pistons with a boss 302 style crown otherwise the canted valves of the 351c heads will hit the pistons.
second you have to plug the coolant transfer passages at the front and rear of the heads otherwise coolant will leak(flood!!) down into the oilpan causing real problems. you also need to make new coolant transfer passages in the manifold face. you can use a boss 302 intake gasket to get the location.
the best intake to use is the old B&A or the new bush performance intakes, but they cost a ton of cash(around $500). you can adapt a 302 style windsor intake, but you need to change a few bolt hole angles, make an adaptor, as well as a couple of other tricks. there is an article in a book published by hot rod magazine that actually details the install. i forget the name of the book off hand. while the swap can be, and has been, done successfully, i highly recomend buying the afr185 heads andbe done with it. you will have about the same money invested either way, but with the afr's you get a lighter, better performing head. your choice though.