You don't say how the car will be used or the performance level you are seeking, but since you're considering a 351W swap I'd guess you want to build some hp.
The 351W will
fit the '65 Mustang engine bay.......However, be prepared for header/hood clearance issues along with other physical differences when using the 351W.
OTOH, you can make decent power for the street with a 302/306 if you match up your parts to work together.
And....don't forget the stroker option.