From most of the dyno charts i've seen over time it appears that all the kits make about the same total HP and torque (a little edge to the 351 on torque) but the larger displacement seems to move the torque and HP curve down lower on the RPM range. IE the 351 puts the power down sooner.


At a certain point beyond about 330cc it seems like the head design is what starts holding back unlocking more power (along with the stock cams). There are ways to address this but by that time you'll already have so much money into the bike its probably cheaper just to get a bigger bike.

I think the 340 is more or less a remnant of Bills development of the 351 kit, seeing how far he could push it. IIRC its still a "wet sleeve" and probably would have little gain over the 331 kit which is a "dry sleeve" and IMO the choice would be either 331 or 351 if you are looking at Bill's kit.