From Novak's webpage about T-18's they say that the bellhousing bolt pattern is the same as the T-150/176, so you are good for that. You already will have the 76-79 clutch disk that is needed (I had to get a new clutch as well).
However, since you have a cj5, mating the D300 to the T-18 will create rear driveshaft issues. I believe that you have a slight advantage since the first year or so the output of the D300 was shorter than in later years. I would plan on a double cardon rear drive shaft if going with a D300.
The Novak adapter plate is 3.2". http://www.novak-adapt.com/catalog/kit_183.htm
The AdvAdp kit is 3.25" . http://www.advanceadapters.com/catalog/016.html.
Something I found online after I did my swap might help you out with a shorter adapter:
http://www.tmcom.com/rockhard/rock1.htm This is from a scout parts place, so I don't know if this would work for a Ford T-18. (I know that the International T-18/T-19 had different input shaft lengths and the output might also be different).