Hi I have a 65 GTO with a 389 tri power 360 HP would like to rebuild and end up with 500 to 600 HP how should I go about this ie. parts

Wow 500 or 600 hp from a 389. Pay attention to details like port & polish the heads or buy aluimium Eldelbrock or Dart racing heads, w/ big valves, hi-lift & long duration racing cam & kit, high compression forged pistons, MSD ignition, get a engine shop to balance the whole assembly, & assemble the short block, add a racing oil pump & pan, high flow water pump, & top it off with a big port aluminum high rise intake manifold & & a big Holley 4bbl carb like 900 CFM's (more than your stock tri-power.

Thanks for the help would like to keep the tri power setup if possible,

Your tri power doesn't flow enough CFM's to make 500 or 600 hp.! It looks cool that's about it. The 1965 intake is old " low rise" tech & new after market Eldelbrock or Dart hi-perf head intake ports are much bigger too. So a new hi-rise aluminum intake manifold needed to match. If you >have to< keep the 3 / 2's dump the Rodchester carbs for bigger 3ea. Holley 500 cfm 2bbl.= 1500 CFM's see?

Just a thought, you can mix & match some 421" motor (cyl heads?)stuff too, they're compatible. If it were me I'd just build a ground pounding 455. The newer 400-455's are a much better design. I had many friends with GTO's but they never beat my '69 big block 396 then to a 427 L-88 SS Camaro.