What is a decent header dimension? obviously the larger the better but what did the stock 351W's come with and what should be the minimum from a performance standpoint?

 

351W engines have pretty small valves so they can only process so much air, most any good tube header will breathe fine with a winsor head unless you have a lot of head mods. tube size of 1 5/8 in. will do ok, but the bigger the tube the more torque, use smalle tubes for higher rpm, but lower torque.