As you WELL know, that '66 FSM is probably the most detailed and explicit of the lot. It taught ME PLENTY about these and even the B body cars, by way of comparison and similarity.
Regardless of how the cleansing occurs, once done, one can see, then repair. Given the amount of dirt described, I figured the car wash engine wash would be the best tool. These do NOT apply at high pressure, though the rinse certainly is. For those who lack experience with these, BE DAMNED SURE TO PUT A HEAVY TRASH BAG OVER THE CARBURETOR, DISTRIBUTOR AND ALTERNATOR before starting the wash.
The car wash can SAVE money and time, used wisely, but it also can CO$$$T a fortune if negligent mistakes get made.
Proper Prior Preparation Prevents Piss Poor Performance.