how to get 340 into 12,s

The compression - was that measured or assumed? Which pistons? HP Holleys are wet flowed, so it's larger than the "normal" 3310s and 750 cfm 4150s. You will need to do some tuning, and tuning starts with timing. Set timing first, then set carb. For what you have, I'd be looking for an inital figure of 18°, centrifical in the 20-22° area, for a total of 38-40. Once you;ve gott aht set, then you can mess with the carb which IMO is too large.