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Top mount (t3 flange) is really the only way for the GT30s to breathe. Heavy Throttle makes a downpipe for the top mount GT30s, but most of the people I know that have top mounts have pieced their stuff together, so they have a custom downpipe made (just make sure you have flex section). You'll have a very hard time hitting 400 without cams, just not a good idea... should be able to see 350 without cams on the stock block but it won't be fun/easy (lots of boost, probably 20psi). Anyway if 450 is your target, you might be able to get away with the GT2871 .86, and that'd be more responsive than either of the GT30s, as well as letting you run bottom mount. Just food for thought.
Also, 550s wont like anything above 400 hp, so start lookin into some bigger sprayers as well, once you build the block.
Another thing, engine tuning isn't cheap, so budget in a good $500 for tuning that beast at a reputable shop.
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