There is nothing like original in my opinion, but if you are going to change the Engine, I would do it right like mentioned above.
But the Disc Brakes are another thing. They were never offered in '66. It would be a toss up between what you want and losing the originality of them. If you do swap, save all the original parts though.
Also, in my opinion, it would sell better original.
Disc brakes appear on modern cars not because they stop better, they don't (more on this in a sec) but because they are cheaper than drum brakes. They appeared on the front only because of the emergency brakes require a drum brake. So for decades the rear brake remained a drum brake. Current four wheel disc cars use the Emergency brake only as a parking brake. Because of the pressure required to stop the rotor from turning the pad size on the e-brake is so small you would hit whatever you were trying to stop from hitting because it takes an airport run way to stop the car with just the e-brake.
Now for that provocative statement about a drum brake being more efficient at stopping a car. Drum brakes are self activating. The shoes want to dig into the drum to stop the drum from turning. It takes two really stiff springs to counter act this force. Because of this drum brakes require half of the pedal pressure to work. With a drum brake you can lock up the brakes at will in a panic stop. That pretty well maxes out the braking effort.
Aside from cost why do race cars and sports cars have disc brakes if drum brakes work better? This is because repetitive braking (such as LeMans, Watkins Glenn, Sebring or even short track roundy-round racing) pump a lot of heat into the brakes. This heat leads to brake fade unless you have carbon fiber rotors and ceramic pads as found on super cars. Disc brakes have their braking surface hanging out in the open to shed heat. They are often vented on race cars and have vanes cast inside that move air from the hub to the out side to increase heat flow. Even so at night you can go to an endurance road race and see the rotors glowing cherry red continuously as the drivers are on the brake and gas, heel to toe all during the race.
I might add that the trucking industry sill used drum brakes on semi trailers and they have no trouble locking up dual and triple axles trying to keep from hitting a dumb four wheeler pulling in front of them.
So how do you improve drum brake performance? Get a bigger drum off of a heavier vehicle, or off one that drove a lot faster such as early Buick and Pontiac finned aluminum drums with wider shoes inside. Station wagons police cars with HD brakes are a good source, as well as five lug pick-up truck brakes from the sixties. You want a wider drum and shoe combination with a heavy drum to absorb the heat of an emergency stop. Even so you have to consider these brakes were designed to stop the car twice from a highway speed of only 45 mph (the national road speed average in 1967). From higher speeds you risk brake fade from heat build up.
One final word about disc brakes. They push the wheels further out increasing track width up to an inch and a half; which can be compensated for by buying custom wheels with reduced back spacing. Because they require nearly twice the pedal effort to work you generally have to go to power assist brakes. And if you retain the drum brakes in the rear you need a proportioning valve to reduce the hit of the brake line pressure to prevent skidding.