Rtoo swap

They will all bolt in the same place. Where is the shift tower on the top plate? Over all length? U-joint size? Mechanical or electric speedo? It can all be overcome.
There are many choices and different ones are better in different applications. The 18 spd is probably the all around winner.
Of a bigger concern is having a package the works together and you can speed a lot of money for nothing. A 6-71 13 spd pulling 80K needed all 13 on a regular basis. Today's higher HP and torque engines do not need to split every high gear unless you are heavy haul and or running mountains. What you want to avoid is buying your own problems. An O/O I worked with had a hot Big Cam III. 11R-24.5's and 3.70's. Took out a direct 9 and installed a RTO1010, 12% OD? Wanted more and installed a RTO12609P? 22% OD? and hated it. Had to shift all the time in rolling hills. A 13 or 18 running one gear down is still in direct, a good place to operate, and the other is there if you need it sometimes.
Buying transmissions and or rear ends to save fuel mileage is almost always an over all loss. If you need a transmission or rear end anyway and make a good change decision that is a different story.
Never saw where you mentioned the model number so no one can tell you.
Just my opinion. Good Luck.