I put a 8.8 in my 87 dakota, Best I remember, the 8.8 was about 1/2 shorter.... I ended up using a new driveshaft because My trans was a little shorter as well....
But as said, the stock driveshaft should be fine with the correct u-joint, but if you want a little longer, you may consider something like this JEGS 60620 Mustang Driveshaft Spacer | eBay