multiply by 16.38 to get the capacity in cc's (i.e 5814cc)- so 5.8 is right
The twin setup would be the best option- saves having to run a set of exhaust pipes all the way from the other side of the engine, with the backpressure differences, heat, and associated headaches that that causes. The 0.68 ones should work well. It will need to be intercooled to achieve that rwkw figure, obviously