Whats the deal with 340s?

340 was rated 275 gross hp and give or take 280-290 gross in reality. Magnums are advertised 250 net hp. The MP crate 360/300hp (gross rating) was a stock Magnum longblock including the camshaft only with a carburetor. The 360 Magnum is the better engine. More torque. More horasepower when rated similarly.
But - the aura around the 340 is much stronger. And as YR said - once you rebuild them as street performers they are very similar, with the 360 getting the knod for torque, and the 340 for horsepower.