Should I bore a 74 318 to 340 ci or just go with a 360?

For high revs. I would just use the 318 bore, boost it with 9.5:1 coml ratio and do a good cam/lifter/spring combo, and reputable machine shop to port and flow test the heads. Im sure or i hope your already looking in to forged bottom end to go with the top end performance your looking for. The factory la shaft style rockers are capable of 6400 rpm as tested and proven by hot rod magazine. And the boosted 318 with 4.10 gears would produce plenty of power and torque to go slide ways on the weekend. JMHO