I do not think it necessarily matters if it is a 66 or 67 if it is kept original, but how clean, complete, or in some way unique it is to start with.
I'm not a fan of getting a beat up pile and buying every single repair panel, reproduction or NOS part you can find and calling it a pristine vehicle when it is finished.
If it is a nice clean rig, or otherwise rare/unique (factory 289 66 U13 or U14, Explorer package with sunburst interior like recently on ebay, Explorer or Special Decor with stripe kit, very low mileage, etc.) would play more to me whether to keep it in original form or not more than just the year.
We had a 77 Ranger that we recently sold which was well worth keeping original more than some earlier ones we had. The same holds true for our current 77 EB (the Balloon Chaser Ambulance).
On the 77 Balloon Chaser F100 I also have, I plan to keep it as near to original as possible with a few exceptions: It now has a 460 instead of the original 351M which I plan to keep in it. It also now has functional dual stack exhaust versus the original faux stacks which I also plan to keep. I may pull the original rear axle and store it away and build up a heavy duty 9" with limited slip, lower gears and disc brakes as I intend for this to perform like the hot rod it looks like and want to be able to lay down some nice burnouts with it when the need/urge arises! ;D