Zion,
It's just my opinion, but I think that a shaft extension would be a big mistake.
I promise you that the closer the prop is to the supporting bearings, the better off you will be.
Maybe I misunderstood your question. 6" between head and prop should be plenty. (At least as far as the head and the prop care)
I don't know the specifics, but I'd go with the cut and re weld the headers idea, I think.
How about posting a couple photos, so we better understand?
Duane
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