Which intake should I use/ buy

The absolute best street dual plane intake for SBM is the rpm air gap. On ~260hp 340 I’d imagine it would pick up around 10hp and 5-10lb/ft or so over the Weiand. Also I’d personally throw the 600 eldlebrock carb in the trash and run a Holley 3310 750vs but that’s just me. Only you can decide if the cost of those things is worth the difference in power. If your current combination is not perfectly tuned then there is probably more power hidden in the tune up then swapping out parts.