"Correct" motor
If you are looking to restore the car to the "correct" motor, the M code 290 hp 351 (assuming it had an "M" code in the VIN) would be correct.
In 1969, I believe that motor WOULD have been made at the Windsor plant (which was just closed down, by the way).
Now, I'm not sure whether in other model years the Windsor plant made a 290hp rated 351 or not - I'll dig around and see. But you should be able to find a 290hp 351 Windsor motor.
Here is the method for identifying the 2 and some data about their differences (which you say you know about, but hey, maybe this will help):
Now a 351 Windsor is really a slightly enlarged 289/302, as it's name implies it comes from Fords "Windsor" engine family (a.k.a. the 90-degree V engine family). The thin-wall cast SMALL BLOCK accepts regular sized spark plugs, uses a timing chain in the block, routes water through the intake manifold, features thin main-bearing caps, a very good oiling system, and uses the same heads for 2V & 4V versions. The heads are are small, utilizing in-line valves with relatively small ports. The valves are 1.78" intake and 1.54" exhaust, i.e. the same size as a 289/302. The valve covers are straight (front to rear), attached by 5 bolts, and when removed you can see 351 cast in the lifter valley. The small side-by side (in-line) valves are the dead give-away.
The 351 Cleveland, on the other hand, belongs to Ford's 335 engine family. This thin-wall cast BIG SMALL BLOCK uses the smaller 14mm spark plugs, has a separate front cover (bolted to the block) housing the timing chain and routing water - so that water does not go through the intake manifold, features beefy main caps (wide enough to drill for 4-bolt mains), a poor oiling system, and uses different heads for 2V & 4V versions. The heads make all the difference and these fire breathing babies make this motor the legend it is. On the 4V, the valves are HUGE, measuring 2.19" intake and 1.7n" exhaust (don't remember exactly). Valves this large are only possible via a canted valve arrangement, forming what Ford refers to as a "poly-angle" combustion chamber. The valve covers are not straight - the front is flat and parallel to the ground, but a curve twists the rear parallel to the head. They are attached by 8-bolts and when removed, there is a 4 cast into the corner of the 4V and a 2 cast into the corner of the 2V (at least in 1970). The canted valves are the dead giveaway.
Summary of Quick ID351 Windsor 351 Cleveland 5-bolt straight valve covers 8 bolt 2-plane valve covers radiator hose to the intake manifold radiator hose does not connect to intake regular 5/8" spark plugs small 14mm spark plugs