Easy? The '67 had a 390/C6 as an option, so you'd be dealing only with factory parts to make the switch. Brakes, everybody's got a disk brake conversion kit. The '67 had an available dual reservoir master cylinder with power booster, which is as good as it gets, and again you'll be dealing with factory parts to get there. You don't really need to swap out the 8" axle, depending on how built your 390 is and how hard you intend to push it.
The Cougar of course has the headlight doors and the sequential turn signals to add complexity, but it's a well-traveled path. Complete interior kits are available too.
You didn't mention rust repair, but you need to check out the cowl vents for leaks -- almost guaranteed that you'll find some -- and the floor pans for rust through, particularly the front seats. The front and rear corners of the doors bubble up, and so do the rear edges of the front fenders and the front edges of the rear quarters. Trunks aren't as big a rust problem as with other makes, probably because the gas tank is the trunk floor and they're galvanized.
Anyway, I'm always saying that the true value of most any project core is actually a negative number when measured against the market value of that car in #2 condition minus what it costs to get a core from here to there. You're lucky to be one who bought right. Now the challenge is to keep it from going upside down on you. Almost impossible to do, even when you start out at $5.
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