HOwdy Maverik:
If you're asking if the 250 will match up with the 200 three speed- the simple answer is "no".
The longer answer is that the 250s block bellmounting end is different than the 200. So is the crank hub. The 250 uses a V8 style bell mounting, bell and tranny mount.
The 200 is smaller in that area and the bell housing is unique to small sixes.
After '67 the 200 got the full syncro 3 spd, called a 3.03, which is what 250s got for a three-speed beginning in '69. This trans is physically much larger than the '66 and earlier 2.77 non-synchro. The 2.77 was never used on a 250. So, the tranny may be the same (you didn't say what year it was), the bell housing-to-block is different between the 200 and the 250.
Adios, David