The 351w is a little taller and wider due to the longer stroke. You really cant tell it by just looking at it but if a car has a 302 in it, than the 351W will usually fit. Because of the wider taller block, The intake will not interchange, neither will the oil pan. And if a car is equiped with a 302 like a mustang for instance, you have to use drop motor mounts, because the motor sits to high, or different headers, because the motor sits to high. As far as the weight goes, Im not sure, but I dont believe that there is a whole lot of difference, but the 351W is heavier. Usually you can make up for most of it with aluminum heads, intake, etc. Hope this helps.
89 5.0 coupe, 650 double pumper, RPM intake,E-303 cam, TFS twisted wedge heads, comp rockers, under drive pulleys, pro 5.0 shifter, A/C delete, BBK full legnth headers, BBK shorty H-PIPE, Mac 3" cat back, sub frame con., 3.73s, lakewood lift bars