340 questions

To know crank diameter, check the bearing markings. StD, +0.010, etc. But I would go with a new complete rotating assembly from a US supplier. Cheap insurance, and the opportunity to select stroke. Many options there. Hard to beat ported X heads if this is what it has, but I do like the weight redeuction of Eddy or TF heads now that they are available. Many advantages to Aluminum heads and weight is the enemy (quoting Mopar Action's Ehrenberger). Even though t 360 is overall a better starting point, I am a 340 person for 54 years. Got my first 340 (and still hae it) in 1969. Raced the snot out of it in SoCal for years and never broke a thing. Ran 13.40's @ 105.38 all the time in stock form. Out pulled a brand new 440+6 Cuda at top end at Irwindale in 1970. That surprised him. He thought I had a Hemi! Jaw dropping moment when he saw the 340.
Who in the US is making and selling "cheap" cranks and rods?

There is nothing wrong with factory 340 cranks, there have been many built that ran hard with either cast or forged cranks. Yes, I'd prefer forged, but for 90% of the street builds the cast crank will work fine.