[olddog]

A good 289 can make 450 Hp, but it will have to spin over 7000 rpm to do it, NA. It would be an ill mannered street engine.

You wouldn't need nearly as radical engine if you stroked a 302 to 347. It still has to be on the radical side to get 450 Hp by 6000 rpm.

Stroke a 351 up towards 400 cid and now it starts getting easier to make power and not be too radical. By an aftermarket 4.125 bore block stroke it to 4" and you have a nice 427 cid that can make plenty of power and still have nice street manors, but that block is going to increase the price shy of $3K by the time you do the machining.

Or go with a FE. You can stroke a 390 out to about 440 ish cid. If you happened to have that block you would be in the same price range as building an aftermarket 351 stroker.

For even more money you can build the all out 427 side oiler. There are aftermarket FE Side Oiler blocks in both cast iron and aluminum. It is common to stroke them out to 482 cid. I believe there are some over bore blocks that allow over 500 cid. A lot of pros and cons to these various blocks. Once you get into these, who built it means a lot. Your going to be in the $12K-$20K range for these engines.

The least expensive big inch Ford that can make huge power is the 385 series 429/460. They will fit in a Cobra, but they look like it would take some skills (shoe horns and magic mirrors) to fit one in there. Not much room to work on it. I hear there are few intakes air cleaner combos that will clear the hood. I have seen several and road in one stroked to 532 cid. Awesome does not come close to describing it.

If you are looking for a used completed Cobra, there are a lot of small blocks around. I would say about as many 351 strokers as 302 variants. You will likely see a fair amount of FE based engines, but few true Side Oilers. Be aware that there are fake Side Oilers out there. You will likely see more 385 series engines than true original Side Oilers.

Personally I would care more about what the car was and how it is built than the engine family. The engine family is secondary to me. They all can be built to make decent power. They can be replaced or rebuilt and there are good builders on this site that can help with that. There are not a lot of people who specialize in fixing poorly built frames, steering, and suspension.