13lbs on a stock bottom end... buy a new shortblock. You'll need it sooner or later. I wouldn't advise more than 10psi... I've seen too many cars pop stock motors. Just can't trust the gas.
EDIT: The flat spot at the end of the HP curve may be something to look at. I'd need all kinds of other charts to figure it out though. The D1 is big enough that the thing should rise all the way to redline... maybe the stock cams are holding it back.
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