If weight is a concern you might consider
a Taurus SHO engine [pretty exotic].
Look at.......
http://members.cox.net/rdgrauman/Healey.html

Stick w/ a 302/5.0.  As long as you're at it, if you feel the cubes are
needed, there are popular 331 & 347 stroker kits for the 5.0.  The
narrower 302 makes for a neat install in the Falcon.

Rob

The 302 is physically the same external diminsions as the
289 which was available in the '65's.  It was a fairly tight
fit.  The width of the 351W is a problem, IIRC.  The 351W
stands about .9" taller and a bit wider than the 289 (302)
engines.  Ford made a fairly decent cast iron tri-y header
for the HiPo 289's which are a bolt-on for a 302 that will
clear with the 302.  The nice thing about them is decent
torque in the midrange instead of way above where most folks
ever go.  IIRC, there is (was) a couple of sources for them
in repro.  I have seen a couple of 351W's in them but, IIRC,
they had to have some metal work done on the towers to get
any sort of working clearance for the headers/manifolds.  I
have also seen a couple of hack jobs where the metal work
was done with a half dozen swings of a sledge hammer.

You might keep in mind that, in those days, Ford used quite
a bit different suspension under 6 cyl engines than they did
on the V8's including steering linkage.  You might want to
check into installing later model front suspension with disc
brakes from a Mustang and R&P steering which makes a neat
installation and drives well.  The original drum brakes were
marginal at best when new.  If you use them, make sure you
have a good booster and use metallic shoe linings.  The
first time you try to bring 'er down from 80+ mph, you'll
know why - the brakes tend to run out toward the far end of
the stop with a stock setup.

You should have a nice ride if you do it right. I enjoyed
mine a lot.  Really screwed up when I traded it for a '53
Ford p/u with a SBC power train on a whim.  That was the
second time I got screwed with a bow tie in a street car and
haven't owned one other the the short track car since.

Good luck

Lugnut

Stroke a 302 like the other poster said.

And stiffen up that body/chassis; it wasn't designed for the torque you're
gonna throw at it.