New hemis are 10.5 compression 205 thermostats and run fine on 87 octane with very short duration cams and brittle pistons. Magnums have better chamber and being stuck on 91+ octane is not a big deal. More compression means more efficency and makes up the differance in price. I would shoot for 10 to 1 with magnums and that is conservative. The LD 340 runs out of steam at 6500 on a 340 and a touch less on a 360 if you had a regular performer you would shift the power down a tad but really not much. It will start makeing power around 2000 where even factory converters are just stalling any how. Get a small tube longest length you can find or some early magnum ex manifolds.