Hey, i se ya want some info on a 340. What are you wanting to do? this is what I did to my 340 magnum in my '73 Roadrunner. Now remember this is awhile ago but should still work great.

This comes out to be a little over a 400.
Mopar performance stroker crank
Eagle H-beam Rods
Total seal rings, will need to file for perfect fit.
The crank will need federal Mogul's Super heavy duty alloy race bearings on both the mains and rods. No modification needed here.
I also used Milodon main studs tourqed to 85 lbs in steps. The four bolt main caps outer bolts to 35lbs.
I used arp bolts on everything!
Diamond racing products for pistons
Crane cam solid rollor 260/266 degree @.50 .630/.630 inch lift on a 108 degree spread.
Milodon single idler gear drive fits under stock timing case
I also went with milodon performance oil pump and a deep sump steel pan
i spent alot of time porting heads and increasing air/ fuel flow. there is now after market heads that you can buy that already has everything done to them. get you a Mopar performance manual. Also check out Edelbrock. they have compltete systems that they have tested that work together.
Rember, take your time and enjoy the results
DCON