Are 586 small block heads any good?

Probably so,I'm not real enamored with the LD340's like I say I have used lots of them over the years and I always considered them to be "Just OK" and thats all. $50 was the average swap meet price on them till about 5 years ago and then they started getting more expensive. Now you can pick up a regular performer RPM for around $100 and they are a much better intake.