Whats the deal with 340s?

moper said
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.

240 net on the low CR 340s, about the same as a 5.2 Magnum. The '68-'71 340s did have more power, probably at least 275 net.

Mopar Tim said
360 Magnum, Lots better than 360 LA.

Old E58 was rated about the same and ran about the same in a similar weight truck as the 360 Mag.

A 1971 340 ran better than a 1974 360, same with a 1971 383, a 1971 440, but in 1974 that wasn't the case. The big blocks and 340s were seriously at a disadvantage compared to 360s in the emissions environments. The big blocks were still faster on the top end, but it was increasingly less relevant. Your '73 340- it'll run about like a 5.2 Magnum. Not a bad motor, add good value to a car it's in, but not a pinnacle in any way. A '73 should be externally balanced too.