I'd say it's a bit much as well, but I'm sure that is spec, they didn't want it to leak. An original cast iron manifold may take it but an aluminium manifold may strip. If I'd been thinking, and not in a big rush, I would have heli-coiled my manifold when I took it out of the box.
Same thing has me worried about my aluminium heads, none of the threaded holes are heli-coiled.
And just last week I had to heli-coil threads in the cast aluminium 68 power steering/alternator bracket. Aluminium is great stuff but work hardens rapidly and is prone to galvanic corrosion. After suffering from belt squeak for years I changed pulleys and belts many many times, the bolt hole in the bracket finally work hardened and stripped out. It was a pain but I managed to drill out and heli-coil it in place.