Re: 390fe How much is too much?
FE blocks will crack. Usually between the crank and the cam. There's no exact power level when this happens. I would say that over 600 hp and over 7000 rpm with a 2 bolt block it becomes likely, at least eventually.
The fix, aside from an aftermarket block, is to cross bolt the mains and possibly to fill or half fill the block with Hard Blok. With a good tune, I'm imagine 900+ hp could last quite a while. It'd last much longer if the power is made at lower rpms with a turbo or supercharger.
There's more to it than that, but if you really want that kind of power in a factory FE block you're talking about a forged crank, good aftermarket rods, crossbolts, and maybe some block fill.
Just FYI, my 2 bolt 428 block already has a half fill of Hard Blok. On my next rebuild I plan on adding Pro Gram Engineering cross bolt caps, a factory forged crank, and aftermarket rods. I have the main caps and crank. I haven't decided on what rods to get.
1967 Cougar. 432 cid FE. C6, 9 inch rear.