twin sticked. not sure what you want to know but there is no twin sticking the 231, just the d300. There are plenty of ppl offering twin stick kits for the 300. The only thing you would be gaining by twin sticking the d300 is being about to put it in front wheel drive (front dig). If i had the money I would go for a triple stick cable shifter (vendors on pirate have them), the cable shifters would make figuring out all the shifter linkage much easier.
Mounting. There is no factory mounting holes in the 231 or the 300 so you will have to figure out something on the dana 300 (I plan on using some of the existing bolts/holes on the rear output shaft housing and fabing up a rear mount to help support all the weight hanging out there).
Dimensions. The 231 part of the doubler adds 7 7/8" (duffy's box 4 rocks). I do not know the length of a d300 but im sure you can google it the same as i can.
Cost. Box-4-rocks kit I got from duffy on pirate for $499.99.