Which is the better motor for a 1970 ford galaxie 500? Boss 302 or 351 Windsor? ?

390 FE for the near win. 427 FE for the win.

Get a 351 Cleveland.

The 302 wouldn't have enough low end torque for a car that heavy.  The 302 used in the 1970 Galaxies was not the Boss 302, it was just a normal plain-jane 2 barrel.   Also neither the Galaxie or the Torino were sports cars. They were far too heavy for that.  The Boss 302 didn't have much low-end power, it was made with top end performance in mind. In a 4000+pound  Galaxie it would be an absolute dog.  

A 351W or even a 408W/427W stroker would be more in character with the car. A 429 would be a fine choice, but a 427W would be far more most cost effective, while easily matching or beating the 429 for power. 

With a car that big you need torque and the best way to get it is with more cubic inches so go for the 351.

I owned the ORIGINAL BOSS 302 engine and it was SUPERB! I disconnected all the SMOG plumbing and had the DISTRIBUTOR recurved and A FULL RACE CAM put in instead of the FACTORY 3/4 RACE CAM! The HEADS are SUPERB and the the BLOCK had FOUR BOLT MAINS! it is VERY DURABLE and FUN FUN F U N <   ! There is ONE THING THOUGH! Somewhere in print is a story about HOW to PREVENT the oil from gathering in the REAR of the engine! It has to do with OIL GALLERIES in the REAR of the engine! I would DUMP the DUAL POINT ignition for a more DIRECT one like a MALLORY that is ADJUSTABLE! The BOSS 302 I had, made 370 HP at the rear wheels and was a TOTAL GAS to drive around! The BOSS 351 is ALSO capable with those Marvy CLEVELAND HEADS< but I never fooled with them or RACED one! Just the BOSS 429 cars and CHEVIES and CHRYSLERS! The BOSS 302 makes MORE power than you will EVER EVER NEED and if you get your HANDS on the NEW BOSS 302 YOU will smile even WIDER! It is UNREAL and has 444 HP! And can take on SUPERCHARGING as WELL! The 302 is a GREAT one to get MORE power from and FORD RACING has MANY items for it!  THE best thing is that the BOSS 302 vs the 351 has VERY HIGH RPM ranges and the BOSS 351 is TOO MUCH STROKE to rev as HIGH! I shifted mine out at 7800- RPM! It was AWEsome! I had HEADERS of course. And TRACTION BARS! The WHOLE CAR in 1969 was ONLY $3600 with TAX and TITLE back them! IF I HAD IT TODAY it would be worth 250,000 AT LEAST! NEWER MUSTANG blocks have OIL SPRAY JETS in them EXCEPT the new BOSS engine! I do NOT know WHY! MAKES ZERO SENSE to me!  I WISH I could be THERE with you for this mod! FORD RACING will help you a LOT!  The WINDSOR head came from CANADA of course, and was NOT as Efficient as the CANTED VALVE head int CLEVELAND BOSS 302 head It did NOT breathe as well  The BOSS 351 used the CLEVELAND canted valve head, called a SEMI HEMI! 

If you don't have to have it stock and want to use factory parts consider a 351W with 351 Cleveland heads (2 brl). While not a direct bolt on, it is a preferred (for performance) set up. This makes a 351 Boss engine. The heads used on the four barrel engine have too big of ports to be effective at the low end. Great for cruising, but kind of a dog at the lower end. The four barrel heads were developed for NASCAR where the cars spend a lot of  time at the higher rpm ranges.

Ford racing catalogues had the details on how to do the swap (reportedly good for another 50 HP). Alternate are "aussie heads" for the Windsor.

This car is huge.  Not sporty.  351 Windsor would move it, but not in a sporty way.   A 460 would give you enough power to spin the tires, but it would not be sporty.

too big of a car for performance

old blokes spend outrageous amounts of money on old cars to reminisce about being young its not to go fast that car will be go fast.

This was the pic of the motor they added.