Yes, a 283, 305, 307, 327 etc will fit. You should be able to use small block motor mounts from 1966
I have a 1966 Mustang with a 302 V8 motor in it, and I'm pretty sure the body styles are the same for the 1964- 1966 Mustangs.
the big block will bolt right to the small block motor mounts but they do make big block motor mounts that you can buy and install on the 66, it lets the engine sit a little lower and further to the drivers side.
First produced by Ford, then sent to Shelby Motor company to be "upgraded". Carol Shelby is credited for making the Shelby Mustang.
I'll sell you my all-original 3.3 liter engine out of a 1966 mustang for $750 If it's original it will be worth more. A larger motor will be in the thousands
Just pull the old engine out and install a new one , you have to change motor mounts or the K-frame if you go to a bigger engine.
I believe that is 3.68 inches for the 200 cubic inch straight 6 cylinder
ford mustang 1966
The top speed for the 1966 Ford Mustang is around 150 to 170
http://auto.howstuffworks.com/1965-1966-ford-mustang-specifications.htm
How fast does the 1966 Ford Mustang Shelby GT 350 do what?
Research has shown that there are a number of options available for purchasing parts for a 1966 Ford Mustang. These parts can be found on eBay, CJ PonyParts, and Dallas Mustang.