Can I put 1961 Mercury 390 heads on a Ford 390?

Winekpedia? That site is a joke. The reason you can't. The MEL in the Mercs had the combustion chamber in the cylinder and the head was flat. So much so that to plane them you have to remove the valves. If you put an MEL head on a FE block the piston will hit the valves and or the head itself.

Sure, you can put any FE head on any FE block.

"3) Head type: low-riser, medium-riser, high-riser, tunnelport, or SOHC. These descriptions actually refer to the intakes used with the heads...a low-riser intake, designed to fit under a low hoodline was the earliest design. The high-riser intake required a bubble in the hood of cars it was installed in for clearance. While the low and medium riser heads could be used in combination with either low or medium riser intakes, the high riser head required a high-riser intake due to the increased height of the intake port. The medium riser's intake port is actually shorter in height, though wider, than the low-riser's port. The high-riser's ports are taller than either the low or medium-riser ports. Low-riser intakes have the carburetor placed relatively low so that the air-fuel mix must follow a more convoluted path to the chamber. A high-riser's intake places the carburetor approximately 6 in (152 mm) higher so the air-fuel mixture has a straighter path to the chamber. The tunnelport and SOHC heads both bolted onto FE blocks of either variety but required their own matching intakes. Within the major head groups, there were also differences in chamber designs, with small chambers, machined chambers and large chambers. The size and type of chamber affected the compression ratio, as well as the overall performance characteristics of the engine."

Is the recipient vehicle also a 1961 model?

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