Hyde/Ernie...the difference is in the heads. The 320 hp uses the Y303 head; the 345 uses the X303 head. Bigger ports, same valves. By the way, they will sell you a motor with either the E-303 cam or the B-303 cam; E signs off at around 4500 rpm but has better lo end torque, and the B gives up at 6000 but makes better hi end hp.
I opted for the B cam...still need to finish tuning (wanted to DRIVE this year) but just tons of fun. I suspect the E motor might be more impressive when it comes to wheelspin but the B motor will keep you grinning when reaching for 6K in fourth.
- see?