Here are some comparison photos, Road Runner starter Vs OEM Buick 3800 V6 T5 manual starter.
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The Road Runner starter is to suit Holden 304/308 V8 however All they really change is the front plate that bolts to the engine block, the motor and reduction unit are the same 1kw unit they use as far as I can tell. I don't have a spare ecotec starter laying about unfortunately but that should give you a idea of what size it is.
But I don't think it helps your clearance issues as it might move the whole thing closer to the block it also lifts it a bit higher which is where I'm guessing you need to create so room.
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