Eddy heads - will I feel a difference on my 340 ?

I personnallyyyy do not think you'll feel the difference. Very little if at best. Use less money and do the X heads.

If your set on the Edel.'s.
Use the ARP head bolts for the Edel. heads and see this thread; http://www.forabodiesonly.com/mopar/showthread.php?t=24282
About Alt. spacing.

All small block stuff is exchangable except early 273 head and intake. Bolt angles will screw you.
Then the no da, crank and pistons oil pan.